/* ==========================================================================
   CALCSOLVER.LIVE — OFFICIAL DESIGN SYSTEM & STYLESHEET
   Brand Promise: Solve accurately. Understand every step.
   ========================================================================== */

:root {
  /* Brand Color Tokens */
  --color-primary: #2563EB;
  --color-primary-hover: #1D4ED8;
  --color-primary-light: #EFF6FF;
  --color-primary-border: #BFDBFE;
  
  --color-bg-main: #FFFFFF;
  --color-bg-secondary: #F8F9FA;
  --color-bg-card: #FFFFFF;
  --color-bg-input: #FFFFFF;
  --color-bg-hover: #F3F4F6;
  
  --color-text-main: #1F2937;
  --color-text-muted: #4B5563;
  --color-text-subtle: #6B7280;
  --color-text-inverse: #FFFFFF;
  
  --color-border: #E5E7EB;
  --color-border-dark: #D1D5DB;
  
  --color-success: #10B981;
  --color-success-bg: #ECFDF5;
  --color-success-border: #A7F3D0;
  
  --color-warning: #F59E0B;
  --color-warning-bg: #FFFBEB;
  
  --color-danger: #EF4444;
  --color-danger-bg: #FEF2F2;
  --color-danger-border: #FECACA;

  /* Typography */
  --font-family: 'Inter', -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --font-mono: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
  
  /* 8px Spacing System */
  --space-1: 4px;
  --space-2: 8px;
  --space-3: 12px;
  --space-4: 16px;
  --space-5: 20px;
  --space-6: 24px;
  --space-8: 32px;
  --space-10: 40px;
  --space-12: 48px;
  --space-16: 64px;
  --space-20: 80px;

  /* Corner Radii (8px-12px) */
  --radius-sm: 6px;
  --radius-md: 8px;
  --radius-lg: 12px;
  --radius-xl: 16px;
  --radius-full: 9999px;

  /* Restrained Shadows */
  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.05);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.07), 0 2px 4px -1px rgba(0, 0, 0, 0.04);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.08), 0 4px 6px -2px rgba(0, 0, 0, 0.03);
  --shadow-modal: 0 20px 25px -5px rgba(0, 0, 0, 0.15), 0 10px 10px -5px rgba(0, 0, 0, 0.04);

  /* Container */
  --container-max: 1200px;
  --transition-fast: 0.15s ease;
  --transition-normal: 0.25s ease;
}

/* Accessible Dark Mode */
[data-theme="dark"] {
  --color-bg-main: #0B0F19;
  --color-bg-secondary: #111827;
  --color-bg-card: #1E293B;
  --color-bg-input: #0F172A;
  --color-bg-hover: #334155;

  --color-text-main: #F9FAFB;
  --color-text-muted: #D1D5DB;
  --color-text-subtle: #9CA3AF;

  --color-border: #334155;
  --color-border-dark: #475569;
  
  --color-primary: #3B82F6;
  --color-primary-hover: #60A5FA;
  --color-primary-light: rgba(59, 130, 246, 0.15);
  --color-primary-border: #1E3A8A;
  
  --color-success-bg: rgba(16, 185, 129, 0.15);
  --color-success-border: #065F46;
  --color-danger-bg: rgba(239, 68, 68, 0.15);
  --color-danger-border: #991B1B;

  --shadow-sm: 0 1px 2px 0 rgba(0, 0, 0, 0.4);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.5);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.6);
}

/* Tactile Calculator Theme Skins */
[data-skin="classic"] {
  --color-primary: #2563EB;
  --color-primary-hover: #1D4ED8;
  --color-primary-light: #EFF6FF;
  --color-primary-border: #BFDBFE;
}
[data-skin="emerald"] {
  --color-primary: #059669;
  --color-primary-hover: #047857;
  --color-primary-light: #ECFDF5;
  --color-primary-border: #A7F3D0;
}
[data-skin="purple"] {
  --color-primary: #7C3AED;
  --color-primary-hover: #6D28D9;
  --color-primary-light: #F5F3FF;
  --color-primary-border: #DDD6FE;
}
[data-skin="rose"] {
  --color-primary: #E11D48;
  --color-primary-hover: #BE123C;
  --color-primary-light: #FFF1F2;
  --color-primary-border: #FECDD3;
}
[data-skin="charcoal"] {
  --color-primary: #374151;
  --color-primary-hover: #1F2937;
  --color-primary-light: #F3F4F6;
  --color-primary-border: #D1D5DB;
}
[data-skin="cyber"] {
  --color-primary: #10B981;
  --color-primary-hover: #059669;
  --color-primary-light: rgba(16, 185, 129, 0.15);
  --color-primary-border: #047857;
}

[data-theme="dark"][data-skin="classic"] {
  --color-primary: #3B82F6;
  --color-primary-hover: #60A5FA;
  --color-primary-light: rgba(59, 130, 246, 0.15);
  --color-primary-border: #1E3A8A;
}
[data-theme="dark"][data-skin="emerald"] {
  --color-primary: #10B981;
  --color-primary-hover: #34D399;
  --color-primary-light: rgba(16, 185, 129, 0.15);
  --color-primary-border: #065F46;
}
[data-theme="dark"][data-skin="purple"] {
  --color-primary: #8B5CF6;
  --color-primary-hover: #A78BFA;
  --color-primary-light: rgba(139, 92, 246, 0.15);
  --color-primary-border: #4C1D95;
}
[data-theme="dark"][data-skin="rose"] {
  --color-primary: #F43F5E;
  --color-primary-hover: #FB7185;
  --color-primary-light: rgba(244, 63, 94, 0.15);
  --color-primary-border: #881337;
}
[data-theme="dark"][data-skin="charcoal"] {
  --color-primary: #9CA3AF;
  --color-primary-hover: #E5E7EB;
  --color-primary-light: rgba(156, 163, 175, 0.15);
  --color-primary-border: #4B5563;
}
[data-theme="dark"][data-skin="cyber"] {
  --color-primary: #00FF66;
  --color-primary-hover: #33FF88;
  --color-primary-light: rgba(0, 255, 102, 0.15);
  --color-primary-border: #008833;
}

/* ==========================================================================
   Base & Reset
   ========================================================================== */

*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  font-size: 16px;
  scroll-behavior: smooth;
  -webkit-text-size-adjust: 100%;
}

body {
  font-family: var(--font-family);
  background-color: var(--color-bg-main);
  color: var(--color-text-main);
  line-height: 1.6;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  transition: background-color var(--transition-normal), color var(--transition-normal);
}

img, svg {
  display: block;
  max-width: 100%;
}

a {
  color: var(--color-primary);
  text-decoration: none;
  transition: color var(--transition-fast);
}

a:hover {
  text-decoration: underline;
  color: var(--color-primary-hover);
}

button, input, select, textarea {
  font: inherit;
  color: inherit;
}

button {
  cursor: pointer;
  border: none;
  background: none;
}

:focus-visible {
  outline: 2px solid var(--color-primary);
  outline-offset: 2px;
}

.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
  border: 0;
}

/* ==========================================================================
   Layout Containers
   ========================================================================== */

.container {
  width: 100%;
  max-width: var(--container-max);
  margin-left: auto;
  margin-right: auto;
  padding-left: var(--space-4);
  padding-right: var(--space-4);
}

.main-content {
  flex: 1 0 auto;
}

.section {
  padding-top: var(--space-12);
  padding-bottom: var(--space-12);
}

.section-secondary {
  background-color: var(--color-bg-secondary);
}

/* ==========================================================================
   Header & Navigation (Section 3)
   ========================================================================== */

.site-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background-color: var(--color-bg-main);
  border-bottom: 1px solid var(--color-border);
  box-shadow: var(--shadow-sm);
  transition: background-color var(--transition-normal), border-color var(--transition-normal);
}

.header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
  height: 64px;
}

.brand-logo {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  font-weight: 800;
  font-size: 1.25rem;
  color: var(--color-text-main);
  text-decoration: none !important;
}

.brand-icon-wrap {
  width: 34px;
  height: 34px;
  background-color: var(--color-primary);
  color: #ffffff;
  border-radius: var(--radius-md);
  display: flex;
  align-items: center;
  justify-content: center;
  box-shadow: 0 2px 4px rgba(37, 99, 235, 0.3);
}

.brand-icon-wrap svg {
  width: 20px;
  height: 20px;
}

.brand-logo .brand-dot {
  color: var(--color-primary);
}

.nav-desktop {
  display: flex;
  align-items: center;
  gap: var(--space-1);
}

.nav-link {
  font-size: 0.9375rem;
  font-weight: 500;
  color: var(--color-text-muted);
  padding: var(--space-2) var(--space-3);
  border-radius: var(--radius-md);
  transition: background-color var(--transition-fast), color var(--transition-fast);
  text-decoration: none !important;
}

.nav-link:hover, .nav-link.active {
  color: var(--color-primary);
  background-color: var(--color-bg-hover);
}

.header-actions {
  display: flex;
  align-items: center;
  gap: var(--space-2);
}

.search-trigger {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  padding: 6px 12px;
  font-size: 0.875rem;
  color: var(--color-text-subtle);
  background-color: var(--color-bg-secondary);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  transition: all var(--transition-fast);
}

.search-trigger:hover {
  border-color: var(--color-border-dark);
  background-color: var(--color-bg-hover);
  color: var(--color-text-main);
}

.search-kbd {
  font-family: var(--font-mono);
  font-size: 0.75rem;
  background-color: var(--color-bg-main);
  border: 1px solid var(--color-border);
  padding: 1px 6px;
  border-radius: var(--radius-sm);
  color: var(--color-text-muted);
}

.theme-toggle-btn {
  width: 36px;
  height: 36px;
  display: flex;
  align-items: center;
  justify-content: center;
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  color: var(--color-text-muted);
  background-color: var(--color-bg-main);
  transition: all var(--transition-fast);
}

.theme-toggle-btn:hover {
  background-color: var(--color-bg-hover);
  color: var(--color-text-main);
}

.mobile-menu-btn {
  display: none;
  width: 38px;
  height: 38px;
  align-items: center;
  justify-content: center;
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  color: var(--color-text-main);
}

/* Mobile Nav Drawer */
.mobile-nav-drawer {
  display: none;
  position: fixed;
  top: 64px;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: var(--color-bg-main);
  z-index: 99;
  padding: var(--space-6);
  flex-direction: column;
  gap: var(--space-2);
  border-top: 1px solid var(--color-border);
  overflow-y: auto;
}

.mobile-nav-drawer.active {
  display: flex;
}

.mobile-nav-drawer .nav-link {
  font-size: 1.125rem;
  padding: var(--space-3) var(--space-4);
}

/* ==========================================================================
   Hero Section (Compact & Direct)
   ========================================================================== */

.hero-section {
  padding-top: var(--space-8);
  padding-bottom: var(--space-6);
  text-align: center;
  background: linear-gradient(180deg, var(--color-bg-secondary) 0%, var(--color-bg-main) 100%);
  border-bottom: 1px solid var(--color-border);
}

.hero-eyebrow {
  display: inline-block;
  font-size: 0.75rem;
  font-weight: 700;
  letter-spacing: 0.08em;
  text-transform: uppercase;
  color: var(--color-primary);
  background-color: var(--color-primary-light);
  border: 1px solid var(--color-primary-border);
  padding: 4px 12px;
  border-radius: var(--radius-full);
  margin-bottom: var(--space-3);
}

.hero-title {
  font-size: 2.25rem;
  font-weight: 800;
  line-height: 1.2;
  letter-spacing: -0.02em;
  color: var(--color-text-main);
  max-width: 800px;
  margin-left: auto;
  margin-right: auto;
  margin-bottom: var(--space-3);
}

.hero-desc {
  font-size: 1.0625rem;
  color: var(--color-text-muted);
  max-width: 680px;
  margin-left: auto;
  margin-right: auto;
  margin-bottom: var(--space-5);
}

.hero-cta-group {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-3);
  flex-wrap: wrap;
}

/* Buttons */
.btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-2);
  font-size: 0.9375rem;
  font-weight: 600;
  padding: 10px 20px;
  border-radius: var(--radius-md);
  border: 1px solid transparent;
  transition: all var(--transition-fast);
  text-decoration: none !important;
  white-space: nowrap;
}

.btn-primary {
  background-color: var(--color-primary);
  color: #ffffff !important;
  box-shadow: 0 2px 4px rgba(37, 99, 235, 0.2);
}

.btn-primary:hover {
  background-color: var(--color-primary-hover);
  box-shadow: 0 4px 6px rgba(37, 99, 235, 0.3);
}

.btn-secondary {
  background-color: var(--color-bg-card);
  color: var(--color-text-main) !important;
  border-color: var(--color-border);
}

.btn-secondary:hover {
  background-color: var(--color-bg-hover);
  border-color: var(--color-border-dark);
}

.btn-sm {
  font-size: 0.8125rem;
  padding: 6px 12px;
}

/* ==========================================================================
   Flagship Math Solver Section (Section 5 & 8)
   ========================================================================== */

.solver-container {
  max-width: 960px;
  margin-left: auto;
  margin-right: auto;
}

.solver-card {
  background-color: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-xl);
  box-shadow: var(--shadow-md);
  padding: var(--space-6);
  margin-bottom: var(--space-8);
}

.solver-input-row {
  display: flex;
  gap: var(--space-2);
  position: relative;
  margin-bottom: var(--space-4);
}

.solver-input {
  flex: 1;
  font-family: var(--font-mono);
  font-size: 1.125rem;
  padding: 14px 18px;
  background-color: var(--color-bg-input);
  border: 2px solid var(--color-border);
  border-radius: var(--radius-lg);
  color: var(--color-text-main);
  transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.solver-input:focus {
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.15);
  outline: none;
}

/* Keypad */
.keypad-container {
  display: grid;
  grid-template-columns: repeat(8, 1fr);
  gap: 6px;
  margin-bottom: var(--space-4);
  background-color: var(--color-bg-secondary);
  padding: var(--space-3);
  border-radius: var(--radius-lg);
  border: 1px solid var(--color-border);
}

.keypad-btn {
  font-family: var(--font-mono);
  font-size: 0.9375rem;
  font-weight: 600;
  padding: 8px 4px;
  background-color: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  color: var(--color-text-main);
  text-align: center;
  transition: all var(--transition-fast);
}

.keypad-btn:hover {
  background-color: var(--color-primary-light);
  border-color: var(--color-primary-border);
  color: var(--color-primary);
}

.keypad-btn.op {
  background-color: var(--color-bg-hover);
  color: var(--color-primary);
}

/* Example chips */
.example-chips {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  flex-wrap: wrap;
  margin-bottom: var(--space-4);
}

.example-chip-label {
  font-size: 0.8125rem;
  font-weight: 600;
  color: var(--color-text-subtle);
}

.example-chip {
  font-family: var(--font-mono);
  font-size: 0.8125rem;
  background-color: var(--color-bg-secondary);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-full);
  padding: 4px 10px;
  color: var(--color-text-muted);
  cursor: pointer;
  transition: all var(--transition-fast);
}

.example-chip:hover {
  border-color: var(--color-primary);
  color: var(--color-primary);
  background-color: var(--color-primary-light);
}

/* Result Box */
.result-card {
  margin-top: var(--space-6);
  border-top: 1px solid var(--color-border);
  padding-top: var(--space-6);
  animation: fadeIn 0.25s ease-in;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(6px); }
  to { opacity: 1; transform: translateY(0); }
}

.result-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: var(--space-3);
}

.result-badge {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  padding: 3px 10px;
  border-radius: var(--radius-full);
}

.result-badge.exact {
  background-color: var(--color-success-bg);
  border: 1px solid var(--color-success-border);
  color: var(--color-success);
}

.result-badge.approx {
  background-color: var(--color-warning-bg);
  border: 1px solid #FDE68A;
  color: #B45309;
}

.result-actions {
  display: flex;
  gap: var(--space-2);
}

.result-main {
  background-color: var(--color-bg-secondary);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-4) var(--space-5);
  margin-bottom: var(--space-4);
  font-size: 1.375rem;
  font-weight: 700;
  color: var(--color-text-main);
  display: flex;
  align-items: center;
  justify-content: space-between;
  overflow-x: auto;
}

/* Steps Breakdown */
.steps-container {
  margin-top: var(--space-4);
  background-color: var(--color-bg-main);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  overflow: hidden;
}

.steps-header {
  background-color: var(--color-bg-secondary);
  padding: var(--space-3) var(--space-4);
  font-size: 0.9375rem;
  font-weight: 700;
  border-bottom: 1px solid var(--color-border);
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.step-item {
  padding: var(--space-3) var(--space-4);
  border-bottom: 1px solid var(--color-border);
  font-size: 0.9375rem;
}

.step-item:last-child {
  border-bottom: none;
}

.step-num {
  font-weight: 700;
  color: var(--color-primary);
  margin-right: var(--space-2);
}

.step-desc {
  color: var(--color-text-muted);
  margin-bottom: 4px;
}

.step-math {
  font-family: var(--font-mono);
  background-color: var(--color-bg-secondary);
  padding: 4px 10px;
  border-radius: var(--radius-sm);
  display: inline-block;
  margin-top: 4px;
  color: var(--color-text-main);
}

/* Graph Container */
.graph-card {
  margin-top: var(--space-5);
  background-color: var(--color-bg-main);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-4);
}

.graph-canvas-wrap {
  width: 100%;
  height: 320px;
  position: relative;
  background-color: #FAFAFA;
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  overflow: hidden;
}

[data-theme="dark"] .graph-canvas-wrap {
  background-color: #0F172A;
}

.graph-canvas-wrap canvas {
  width: 100%;
  height: 100%;
}

/* ==========================================================================
   Grids & Cards (Sections 5, 6, 7)
   ========================================================================== */

.section-header {
  text-align: center;
  max-width: 720px;
  margin-left: auto;
  margin-right: auto;
  margin-bottom: var(--space-8);
}

.section-eyebrow {
  font-size: 0.8125rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--color-primary);
  margin-bottom: var(--space-2);
}

.section-title {
  font-size: 1.875rem;
  font-weight: 800;
  letter-spacing: -0.02em;
  color: var(--color-text-main);
  margin-bottom: var(--space-2);
}

.section-desc {
  font-size: 1rem;
  color: var(--color-text-muted);
}

.grid-3 {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: var(--space-6);
}

.grid-4 {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: var(--space-5);
}

.grid-2 {
  display: grid;
  grid-template-columns: repeat(2, 1fr);
  gap: var(--space-6);
}

.tool-card {
  background-color: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-5);
  display: flex;
  flex-direction: column;
  transition: transform var(--transition-fast), border-color var(--transition-fast), box-shadow var(--transition-fast);
  text-decoration: none !important;
  color: inherit;
}

.tool-card:hover {
  transform: translateY(-2px);
  border-color: var(--color-primary-border);
  box-shadow: var(--shadow-md);
}

.tool-card-icon {
  width: 44px;
  height: 44px;
  border-radius: var(--radius-md);
  background-color: var(--color-primary-light);
  color: var(--color-primary);
  display: flex;
  align-items: center;
  justify-content: center;
  margin-bottom: var(--space-3);
  font-weight: 700;
  font-size: 1.25rem;
}

.tool-card-title {
  font-size: 1.125rem;
  font-weight: 700;
  color: var(--color-text-main);
  margin-bottom: var(--space-1);
}

.tool-card-desc {
  font-size: 0.875rem;
  color: var(--color-text-muted);
  flex-grow: 1;
  margin-bottom: var(--space-3);
  line-height: 1.5;
}

.tool-card-cta {
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--color-primary);
  display: inline-flex;
  align-items: center;
  gap: 4px;
}

/* Category Sections */
.category-card {
  background-color: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-xl);
  padding: var(--space-6);
}

.category-title {
  font-size: 1.25rem;
  font-weight: 700;
  margin-bottom: var(--space-4);
  display: flex;
  align-items: center;
  gap: var(--space-2);
  color: var(--color-text-main);
}

.category-links {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
}

.category-link-item a {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-2) var(--space-3);
  border-radius: var(--radius-md);
  font-size: 0.9375rem;
  color: var(--color-text-muted);
  text-decoration: none !important;
  transition: all var(--transition-fast);
}

.category-link-item a:hover {
  background-color: var(--color-bg-hover);
  color: var(--color-primary);
}

/* Feature & Benefit Items */
.feature-box {
  background-color: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-5);
  text-align: left;
}

.feature-box-icon {
  font-size: 1.75rem;
  margin-bottom: var(--space-2);
}

.feature-box-title {
  font-size: 1.125rem;
  font-weight: 700;
  margin-bottom: var(--space-2);
  color: var(--color-text-main);
}

.feature-box-desc {
  font-size: 0.9375rem;
  color: var(--color-text-muted);
}

/* How to use steps */
.steps-row {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: var(--space-6);
  position: relative;
}

.how-step {
  text-align: center;
  padding: var(--space-6);
  background-color: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
}

.how-step-num {
  width: 40px;
  height: 40px;
  margin: 0 auto var(--space-3);
  background-color: var(--color-primary);
  color: #fff;
  border-radius: var(--radius-full);
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: 800;
}

/* FAQ Accordion */
.faq-list {
  max-width: 800px;
  margin-left: auto;
  margin-right: auto;
  display: flex;
  flex-direction: column;
  gap: var(--space-3);
}

.faq-item {
  background-color: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  overflow: hidden;
}

.faq-question {
  width: 100%;
  text-align: left;
  padding: var(--space-4) var(--space-5);
  font-size: 1.0625rem;
  font-weight: 600;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: var(--space-3);
  color: var(--color-text-main);
}

.faq-question:hover {
  background-color: var(--color-bg-hover);
}

.faq-answer {
  display: none;
  padding: 0 var(--space-5) var(--space-4);
  font-size: 0.9375rem;
  color: var(--color-text-muted);
  line-height: 1.6;
}

.faq-item.open .faq-answer {
  display: block;
}

.faq-item.open .faq-icon {
  transform: rotate(180deg);
}

.faq-icon {
  transition: transform var(--transition-fast);
}

/* Breadcrumbs */
.breadcrumbs {
  display: flex;
  align-items: center;
  gap: var(--space-2);
  font-size: 0.875rem;
  color: var(--color-text-subtle);
  margin-bottom: var(--space-4);
  flex-wrap: wrap;
}

.breadcrumbs a {
  color: var(--color-text-muted);
  text-decoration: none;
}

.breadcrumbs a:hover {
  color: var(--color-primary);
}

/* Dedicated Calculator Layout */
.calc-header {
  margin-bottom: var(--space-6);
}

.calc-title {
  font-size: 2.25rem;
  font-weight: 800;
  letter-spacing: -0.02em;
  color: var(--color-text-main);
  margin-bottom: var(--space-2);
}

.calc-intro {
  font-size: 1.125rem;
  color: var(--color-text-muted);
  max-width: 800px;
}

.calc-form-card {
  background-color: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-xl);
  padding: var(--space-6);
  box-shadow: var(--shadow-sm);
  margin-bottom: var(--space-8);
}

.form-group {
  margin-bottom: var(--space-4);
}

.form-label {
  display: block;
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--color-text-main);
  margin-bottom: var(--space-1);
}

.form-control {
  width: 100%;
  font-family: var(--font-mono);
  font-size: 1rem;
  padding: 10px 14px;
  background-color: var(--color-bg-input);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  color: var(--color-text-main);
  transition: border-color var(--transition-fast), box-shadow var(--transition-fast);
}

.form-control:focus {
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px rgba(37, 99, 235, 0.15);
  outline: none;
}

.form-row-2 {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: var(--space-4);
}

.form-row-3 {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  gap: var(--space-4);
}

.calc-prose {
  max-width: 800px;
  margin-top: var(--space-8);
}

.calc-prose h2 {
  font-size: 1.5rem;
  font-weight: 800;
  color: var(--color-text-main);
  margin-top: var(--space-8);
  margin-bottom: var(--space-3);
}

.calc-prose h3 {
  font-size: 1.125rem;
  font-weight: 700;
  color: var(--color-text-main);
  margin-top: var(--space-5);
  margin-bottom: var(--space-2);
}

.calc-prose p, .calc-prose ul, .calc-prose ol {
  color: var(--color-text-muted);
  margin-bottom: var(--space-4);
  font-size: 1rem;
  line-height: 1.7;
}

.calc-prose ul, .calc-prose ol {
  padding-left: var(--space-5);
}

.calc-prose li {
  margin-bottom: var(--space-1);
}

.formula-card {
  background-color: var(--color-bg-secondary);
  border-left: 4px solid var(--color-primary);
  padding: var(--space-4) var(--space-5);
  border-radius: 0 var(--radius-md) var(--radius-md) 0;
  margin: var(--space-4) 0;
}

/* ==========================================================================
   Search Modal (Ctrl+K)
   ========================================================================== */

.search-modal-backdrop {
  display: none;
  position: fixed;
  inset: 0;
  background-color: rgba(0, 0, 0, 0.5);
  backdrop-filter: blur(4px);
  z-index: 200;
  align-items: flex-start;
  justify-content: center;
  padding-top: 10vh;
  padding-left: var(--space-4);
  padding-right: var(--space-4);
}

.search-modal-backdrop.open {
  display: flex;
}

.search-modal-box {
  background-color: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-xl);
  width: 100%;
  max-width: 640px;
  box-shadow: var(--shadow-modal);
  overflow: hidden;
  animation: modalScale 0.15s ease-out;
}

@keyframes modalScale {
  from { opacity: 0; transform: scale(0.97); }
  to { opacity: 1; transform: scale(1); }
}

.search-modal-header {
  display: flex;
  align-items: center;
  gap: var(--space-3);
  padding: var(--space-3) var(--space-4);
  border-bottom: 1px solid var(--color-border);
}

.search-modal-input {
  flex: 1;
  border: none;
  font-size: 1.125rem;
  background: transparent;
  color: var(--color-text-main);
  outline: none;
}

.search-modal-results {
  max-height: 380px;
  overflow-y: auto;
  padding: var(--space-2);
}

.search-result-item {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-3) var(--space-4);
  border-radius: var(--radius-md);
  color: var(--color-text-main);
  text-decoration: none !important;
  transition: background-color var(--transition-fast);
}

.search-result-item:hover, .search-result-item.selected {
  background-color: var(--color-bg-hover);
  color: var(--color-primary);
}

.search-result-cat {
  font-size: 0.75rem;
  font-weight: 600;
  text-transform: uppercase;
  color: var(--color-text-subtle);
}

/* ==========================================================================
   Footer (Section 21)
   ========================================================================== */

.site-footer {
  background-color: var(--color-bg-secondary);
  border-top: 1px solid var(--color-border);
  padding-top: var(--space-12);
  padding-bottom: var(--space-8);
  margin-top: auto;
}

.footer-grid {
  display: grid;
  grid-template-columns: 2fr 1fr 1fr 1fr 1fr;
  gap: var(--space-8);
  margin-bottom: var(--space-10);
}

.footer-brand p {
  font-size: 0.9375rem;
  color: var(--color-text-muted);
  margin-top: var(--space-3);
  max-width: 300px;
  line-height: 1.5;
}

.footer-title {
  font-size: 0.875rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--color-text-main);
  margin-bottom: var(--space-3);
}

.footer-nav {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: var(--space-2);
}

.footer-nav a {
  font-size: 0.875rem;
  color: var(--color-text-muted);
  text-decoration: none !important;
  transition: color var(--transition-fast);
}

.footer-nav a:hover {
  color: var(--color-primary);
}

.footer-bottom {
  display: flex;
  align-items: center;
  justify-content: space-between;
  border-top: 1px solid var(--color-border);
  padding-top: var(--space-6);
  font-size: 0.875rem;
  color: var(--color-text-subtle);
  flex-wrap: wrap;
  gap: var(--space-4);
}

/* ==========================================================================
   Responsive Breakpoints
   ========================================================================== */

@media (max-width: 1024px) {
  .grid-4 {
    grid-template-columns: repeat(2, 1fr);
  }
  .grid-3 {
    grid-template-columns: repeat(2, 1fr);
  }
  .footer-grid {
    grid-template-columns: repeat(3, 1fr);
  }
  .footer-brand {
    grid-column: span 3;
  }
}

@media (max-width: 768px) {
  .nav-desktop, .search-trigger {
    display: none;
  }
  .mobile-menu-btn {
    display: inline-flex;
  }
  .hero-title {
    font-size: 1.75rem;
  }
  .grid-3, .grid-4, .grid-2 {
    grid-template-columns: 1fr;
  }
  .steps-row {
    grid-template-columns: 1fr;
  }
  .keypad-container {
    grid-template-columns: repeat(4, 1fr);
  }
  .form-row-2, .form-row-3 {
    grid-template-columns: 1fr;
  }
  .footer-grid {
    grid-template-columns: 1fr;
  }
  .footer-brand {
    grid-column: span 1;
  }
}

/* ==========================================================================
   Code Mode & Interactive Logic Workbench System
   ========================================================================== */

.solver-top-bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: var(--space-2);
  flex-wrap: wrap;
  gap: var(--space-2);
}

.code-mode-switch-wrap {
  display: inline-flex;
  align-items: center;
  gap: var(--space-2);
  background-color: var(--color-bg-secondary);
  border: 1px solid var(--color-border);
  padding: 4px 12px;
  border-radius: var(--radius-full);
  transition: all var(--transition-fast);
}

.code-mode-switch-wrap:hover {
  border-color: var(--color-primary-border);
}

.code-mode-label {
  font-size: 0.8125rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--color-text-muted);
  user-select: none;
}

/* Switch UI */
.switch {
  position: relative;
  display: inline-block;
  width: 38px;
  height: 20px;
}

.switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

.slider {
  position: absolute;
  cursor: pointer;
  inset: 0;
  background-color: var(--color-border-dark);
  transition: .3s;
  border-radius: 20px;
}

.slider:before {
  position: absolute;
  content: "";
  height: 14px;
  width: 14px;
  left: 3px;
  bottom: 3px;
  background-color: white;
  transition: .3s;
  border-radius: 50%;
  box-shadow: 0 1px 3px rgba(0,0,0,0.2);
}

input:checked + .slider {
  background-color: var(--color-primary);
}

input:focus + .slider {
  box-shadow: 0 0 1px var(--color-primary);
}

input:checked + .slider:before {
  transform: translateX(18px);
}

/* Code Mode Active Banner */
.code-mode-indicator {
  display: flex;
  align-items: center;
  justify-content: space-between;
  background: linear-gradient(90deg, rgba(37, 99, 235, 0.12) 0%, rgba(16, 185, 129, 0.1) 100%);
  border: 1px solid var(--color-primary-border);
  padding: 8px 16px;
  border-radius: var(--radius-lg);
  margin-bottom: var(--space-3);
  font-size: 0.8125rem;
  font-weight: 700;
  color: var(--color-primary);
  animation: fadeIn 0.2s ease-in;
}

.code-mode-pulse {
  display: inline-block;
  width: 8px;
  height: 8px;
  background-color: var(--color-primary);
  border-radius: 50%;
  margin-right: 8px;
  box-shadow: 0 0 0 0 rgba(37, 99, 235, 0.7);
  animation: pulseDot 1.5s infinite;
}

@keyframes pulseDot {
  0% { transform: scale(0.95); box-shadow: 0 0 0 0 rgba(37, 99, 235, 0.7); }
  70% { transform: scale(1); box-shadow: 0 0 0 6px rgba(37, 99, 235, 0); }
  100% { transform: scale(0.95); box-shadow: 0 0 0 0 rgba(37, 99, 235, 0); }
}

.code-mode-input {
  letter-spacing: 6px !important;
  font-size: 1.375rem !important;
  font-weight: 800 !important;
  text-align: center !important;
  background-color: var(--color-bg-secondary) !important;
  border-color: var(--color-primary) !important;
}

/* Interactive Logic Workbench Modal */
.logic-modal-backdrop {
  display: none;
  position: fixed;
  inset: 0;
  background-color: rgba(11, 15, 25, 0.85);
  backdrop-filter: blur(8px);
  z-index: 300;
  align-items: center;
  justify-content: center;
  padding: var(--space-4);
  overflow-y: auto;
}

.logic-modal-backdrop.open {
  display: flex;
}

.logic-modal-box {
  background-color: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-xl);
  width: 100%;
  max-width: 880px;
  max-height: 90vh;
  box-shadow: var(--shadow-modal);
  display: flex;
  flex-direction: column;
  overflow: hidden;
  animation: modalScale 0.2s ease-out;
}

.logic-modal-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-4) var(--space-6);
  border-bottom: 1px solid var(--color-border);
  background: linear-gradient(90deg, var(--color-bg-secondary) 0%, var(--color-bg-card) 100%);
}

.logic-header-badge {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  font-size: 0.6875rem;
  font-weight: 800;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  padding: 3px 8px;
  border-radius: var(--radius-full);
  background-color: var(--color-success-bg);
  border: 1px solid var(--color-success-border);
  color: var(--color-success);
}

.logic-modal-tabs {
  display: flex;
  border-bottom: 1px solid var(--color-border);
  background-color: var(--color-bg-secondary);
  overflow-x: auto;
}

.logic-tab-btn {
  padding: var(--space-3) var(--space-5);
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--color-text-muted);
  border-bottom: 2px solid transparent;
  white-space: nowrap;
  transition: all var(--transition-fast);
}

.logic-tab-btn:hover {
  color: var(--color-primary);
  background-color: var(--color-bg-hover);
}

.logic-tab-btn.active {
  color: var(--color-primary);
  border-bottom-color: var(--color-primary);
  background-color: var(--color-bg-card);
}

.logic-modal-content {
  padding: var(--space-6);
  overflow-y: auto;
  flex: 1 1 auto;
}

.logic-panel {
  display: none;
}

.logic-panel.active {
  display: block;
}

.puzzle-card {
  background-color: var(--color-bg-secondary);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-5);
  margin-bottom: var(--space-4);
}

.puzzle-title {
  font-size: 1.125rem;
  font-weight: 700;
  color: var(--color-text-main);
  margin-bottom: var(--space-2);
  display: flex;
  align-items: center;
  gap: 8px;
}

.puzzle-desc {
  font-size: 0.9375rem;
  color: var(--color-text-muted);
  margin-bottom: var(--space-4);
  line-height: 1.6;
}

.code-shortcut-table {
  width: 100%;
  border-collapse: collapse;
  margin-top: 12px;
  font-size: 0.875rem;
}

.code-shortcut-table th, .code-shortcut-table td {
  padding: 10px 14px;
  border-bottom: 1px solid var(--color-border);
  text-align: left;
}

.code-shortcut-table th {
  background-color: var(--color-bg-secondary);
  font-weight: 700;
  color: var(--color-text-main);
}

.code-shortcut-table code {
  font-family: var(--font-mono);
  font-weight: 700;
  color: var(--color-primary);
  background-color: var(--color-primary-light);
  padding: 2px 8px;
  border-radius: var(--radius-sm);
}

/* ==========================================================================
   Tactile Skin Swatch Picker
   ========================================================================== */
.theme-skin-picker {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  background: var(--color-bg-secondary);
  border: 1px solid var(--color-border);
  padding: 3px 8px;
  border-radius: var(--radius-full);
}

.theme-skin-label {
  font-size: 0.6875rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.04em;
  color: var(--color-text-subtle);
  margin-right: 2px;
}

.theme-skin-dot {
  width: 16px;
  height: 16px;
  border-radius: 50%;
  border: 2px solid transparent;
  cursor: pointer;
  padding: 0;
  transition: transform var(--transition-fast), border-color var(--transition-fast);
  outline: none;
}

.theme-skin-dot:hover {
  transform: scale(1.25);
}

.theme-skin-dot.active {
  border-color: var(--color-text-main);
  transform: scale(1.15);
  box-shadow: 0 0 0 2px var(--color-bg-card);
}

/* ==========================================================================
   Ruled Paper Scratchpad (Signature Feature)
   ========================================================================== */
.scratchpad-card {
  margin-top: var(--space-6);
  background: var(--color-bg-card);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  box-shadow: var(--shadow-sm);
  overflow: hidden;
  transition: box-shadow var(--transition-normal);
}

.scratchpad-card:hover {
  box-shadow: var(--shadow-md);
}

.scratchpad-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 10px 16px;
  background: var(--color-bg-secondary);
  border-bottom: 1px solid var(--color-border);
  flex-wrap: wrap;
  gap: 8px;
}

.scratchpad-title-wrap {
  display: flex;
  align-items: center;
  gap: 8px;
}

.scratchpad-title {
  font-weight: 700;
  font-size: 0.875rem;
  color: var(--color-text-main);
  letter-spacing: -0.01em;
}

.scratchpad-badge {
  font-size: 0.6875rem;
  font-weight: 600;
  padding: 2px 7px;
  border-radius: var(--radius-full);
  background: var(--color-success-bg);
  color: var(--color-success);
  border: 1px solid var(--color-success-border);
  display: inline-block;
}

.scratchpad-actions {
  display: flex;
  gap: 6px;
  flex-wrap: wrap;
}

.scratchpad-actions .btn {
  padding: 4px 10px;
  font-size: 0.75rem;
  font-weight: 600;
}

.scratchpad-body {
  position: relative;
  background-color: #FCFDFD;
}

[data-theme="dark"] .scratchpad-body {
  background-color: #0F172A;
}

.ruled-margin {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 48px;
  width: 2px;
  background: rgba(239, 68, 68, 0.28);
  pointer-events: none;
  z-index: 2;
}

.ruled-textarea {
  width: 100%;
  min-height: 160px;
  padding: 14px 16px 14px 58px;
  border: none;
  resize: vertical;
  font-family: var(--font-mono);
  font-size: 0.875rem;
  line-height: 28px;
  background-color: transparent;
  background-image: repeating-linear-gradient(
    transparent,
    transparent 27px,
    rgba(37, 99, 235, 0.09) 27px,
    rgba(37, 99, 235, 0.09) 28px
  );
  color: var(--color-text-main);
  outline: none;
  display: block;
}

[data-theme="dark"] .ruled-textarea {
  background-image: repeating-linear-gradient(
    transparent,
    transparent 27px,
    rgba(255, 255, 255, 0.06) 27px,
    rgba(255, 255, 255, 0.06) 28px
  );
}

.ruled-textarea:focus {
  outline: none;
}

/* Print Styles */
@media print {
  .site-header, .site-footer, .search-trigger, .theme-toggle-btn, .keypad-container, .example-chips, .hero-cta-group, .code-mode-switch-wrap, .code-mode-indicator {
    display: none !important;
  }
  body {
    background: #fff !important;
    color: #000 !important;
  }
  .solver-card, .calc-form-card {
    border: none !important;
    box-shadow: none !important;
  }
}

/* ==========================================================================
   CALCSOLVER BLUE BRANDING OVERRIDES
   ========================================================================== */
:root {
  --brand-green: #2563eb !important;
  --brand-emerald: #2563eb !important;
  --cs-brand-green: #2563eb !important;
  --cs-brand-blue: #2563eb !important;
  --primary-color: #2563eb !important;
  --color-primary: #2563eb !important;
  --color-primary-hover: #1d4ed8 !important;
  --color-primary-light: #eff6ff !important;
  --color-primary-border: #bfdbfe !important;
}

.cs-pwa-nav-btn {
  display: none !important;
}

.calcsolver-brand-icon {
  background: linear-gradient(135deg, #2563eb, #1d4ed8) !important;
  color: #ffffff !important;
}

.btn-primary, .calc-btn, button.btn-solve, .solve-btn {
  background: #2563eb !important;
  border-color: #2563eb !important;
  color: #ffffff !important;
}

.btn-primary:hover, .calc-btn:hover {
  background: #1d4ed8 !important;
  border-color: #1d4ed8 !important;
}

.ah-badge, .breadcrumb-item.active, .cat-badge {
  color: #2563eb !important;
  background-color: #eff6ff !important;
  border-color: #bfdbfe !important;
}

.cs-menu-toggle {
  display: none !important;
}