:root {
  /* Primitive Color Tokens */
  --color-white: rgba(255, 255, 255, 1);
  --color-black: rgba(0, 0, 0, 1);
  --color-cream-50: rgba(252, 252, 249, 1);
  --color-cream-100: rgba(255, 255, 253, 1);
  --color-gray-200: rgba(245, 245, 245, 1);
  --color-gray-300: rgba(167, 169, 169, 1);
  --color-gray-400: rgba(119, 124, 124, 1);
  --color-slate-500: rgba(98, 108, 113, 1);
  --color-brown-600: rgba(94, 82, 64, 1);
  --color-charcoal-700: rgba(31, 33, 33, 1);
  --color-charcoal-800: rgba(38, 40, 40, 1);
  --color-slate-900: rgba(19, 52, 59, 1);

  /* JS Yellow Accent */
  --color-js-yellow: #f7df1e;
  --color-js-yellow-dark: #c4b018;
  --color-js-yellow-light: #fff8c4;

  /* Teal palette */
  --color-teal-300: rgba(50, 184, 198, 1);
  --color-teal-400: rgba(45, 166, 178, 1);
  --color-teal-500: rgba(33, 128, 141, 1);
  --color-teal-600: rgba(29, 116, 128, 1);
  --color-teal-700: rgba(26, 104, 115, 1);
  --color-teal-800: rgba(41, 150, 161, 1);
  --color-red-400: rgba(255, 84, 89, 1);
  --color-red-500: rgba(192, 21, 47, 1);
  --color-orange-400: rgba(230, 129, 97, 1);
  --color-orange-500: rgba(168, 75, 47, 1);

  /* RGB versions */
  --color-brown-600-rgb: 94, 82, 64;
  --color-teal-500-rgb: 33, 128, 141;
  --color-slate-900-rgb: 19, 52, 59;
  --color-slate-500-rgb: 98, 108, 113;

  /* Background color tokens (Light Mode) */
  --color-bg-1: rgba(59, 130, 246, 0.08);
  --color-bg-2: rgba(245, 158, 11, 0.08);
  --color-bg-3: rgba(34, 197, 94, 0.08);
  --color-bg-4: rgba(239, 68, 68, 0.08);
  --color-bg-5: rgba(147, 51, 234, 0.08);
  --color-bg-6: rgba(249, 115, 22, 0.08);
  --color-bg-7: rgba(236, 72, 153, 0.08);
  --color-bg-8: rgba(6, 182, 212, 0.08);

  /* Semantic Color Tokens (Light Mode) */
  --color-background: var(--color-cream-50);
  --color-surface: var(--color-cream-100);
  --color-text: var(--color-slate-900);
  --color-text-secondary: var(--color-slate-500);
  --color-primary: var(--color-teal-500);
  --color-primary-hover: var(--color-teal-600);
  --color-primary-active: var(--color-teal-700);
  --color-secondary: rgba(var(--color-brown-600-rgb), 0.12);
  --color-secondary-hover: rgba(var(--color-brown-600-rgb), 0.2);
  --color-secondary-active: rgba(var(--color-brown-600-rgb), 0.25);
  --color-border: rgba(var(--color-brown-600-rgb), 0.2);
  --color-btn-primary-text: var(--color-cream-50);
  --color-card-border: rgba(var(--color-brown-600-rgb), 0.12);
  --color-error: var(--color-red-500);
  --color-success: var(--color-teal-500);
  --color-warning: var(--color-orange-500);
  --color-info: var(--color-slate-500);
  --color-focus-ring: rgba(var(--color-teal-500-rgb), 0.4);

  /* Code block colors (Light) */
  --color-code-bg: #1e1e2e;
  --color-code-text: #cdd6f4;
  --color-code-toolbar: #181825;
  --color-code-border: rgba(var(--color-teal-500-rgb), 0.3);

  /* Typography */
  --font-family-base: "Onest", "Inter", -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  --font-family-mono: "JetBrains Mono", ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, monospace;
  --font-size-xs: 11px;
  --font-size-sm: 12px;
  --font-size-base: 14px;
  --font-size-md: 14px;
  --font-size-lg: 16px;
  --font-size-xl: 18px;
  --font-size-2xl: 20px;
  --font-size-3xl: 24px;
  --font-size-4xl: 30px;
  --font-weight-normal: 400;
  --font-weight-medium: 500;
  --font-weight-semibold: 550;
  --font-weight-bold: 600;
  --line-height-tight: 1.2;
  --line-height-normal: 1.5;
  --letter-spacing-tight: -0.01em;

  /* Spacing */
  --space-0: 0;
  --space-1: 1px;
  --space-2: 2px;
  --space-4: 4px;
  --space-6: 6px;
  --space-8: 8px;
  --space-10: 10px;
  --space-12: 12px;
  --space-16: 16px;
  --space-20: 20px;
  --space-24: 24px;
  --space-32: 32px;

  /* Border Radius */
  --radius-sm: 6px;
  --radius-base: 8px;
  --radius-md: 10px;
  --radius-lg: 12px;
  --radius-full: 9999px;

  /* Shadows */
  --shadow-xs: 0 1px 2px rgba(0, 0, 0, 0.02);
  --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.04), 0 1px 2px rgba(0, 0, 0, 0.02);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.04), 0 2px 4px -1px rgba(0, 0, 0, 0.02);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.04), 0 4px 6px -2px rgba(0, 0, 0, 0.02);

  /* Animation */
  --duration-fast: 150ms;
  --duration-normal: 250ms;
  --ease-standard: cubic-bezier(0.16, 1, 0.3, 1);
}

/* ================================
   DARK MODE
   ================================ */
html.dark {
  --color-bg-1: rgba(59, 130, 246, 0.15);
  --color-bg-2: rgba(245, 158, 11, 0.15);
  --color-bg-3: rgba(34, 197, 94, 0.15);
  --color-bg-4: rgba(239, 68, 68, 0.15);
  --color-bg-5: rgba(147, 51, 234, 0.15);
  --color-bg-6: rgba(249, 115, 22, 0.15);
  --color-bg-7: rgba(236, 72, 153, 0.15);
  --color-bg-8: rgba(6, 182, 212, 0.15);

  --color-background: #0f1419;
  --color-surface: #1a1f26;
  --color-text: #e7e9ea;
  --color-text-secondary: #8b98a5;
  --color-primary: var(--color-teal-400);
  --color-primary-hover: var(--color-teal-300);
  --color-primary-active: var(--color-teal-500);
  --color-secondary: rgba(255, 255, 255, 0.08);
  --color-secondary-hover: rgba(255, 255, 255, 0.12);
  --color-secondary-active: rgba(255, 255, 255, 0.16);
  --color-border: rgba(255, 255, 255, 0.15);
  --color-btn-primary-text: #0f1419;
  --color-card-border: rgba(255, 255, 255, 0.1);

  --color-code-bg: #11111b;
  --color-code-text: #cdd6f4;
  --color-code-toolbar: #0a0a14;
  --color-code-border: rgba(255, 255, 255, 0.1);

  --shadow-xs: 0 1px 2px rgba(0, 0, 0, 0.3);
  --shadow-sm: 0 1px 3px rgba(0, 0, 0, 0.4), 0 1px 2px rgba(0, 0, 0, 0.3);
  --shadow-md: 0 4px 6px -1px rgba(0, 0, 0, 0.4), 0 2px 4px -1px rgba(0, 0, 0, 0.3);
  --shadow-lg: 0 10px 15px -3px rgba(0, 0, 0, 0.4), 0 4px 6px -2px rgba(0, 0, 0, 0.3);
}

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

body {
  font-family: var(--font-family-base);
  font-size: var(--font-size-base);
  line-height: var(--line-height-normal);
  color: var(--color-text);
  background-color: var(--color-background);
  overflow-x: hidden;
}

/* ================================
   CUSTOM SCROLLBAR (subtle)
   ================================ */
::-webkit-scrollbar {
  width: 6px;
  height: 6px;
}

::-webkit-scrollbar-track {
  background: transparent;
}

::-webkit-scrollbar-thumb {
  background: rgba(128, 128, 128, 0.25);
  border-radius: var(--radius-full);
}

::-webkit-scrollbar-thumb:hover {
  background: rgba(128, 128, 128, 0.45);
}

html.dark ::-webkit-scrollbar-thumb {
  background: rgba(255, 255, 255, 0.12);
}

html.dark ::-webkit-scrollbar-thumb:hover {
  background: rgba(255, 255, 255, 0.25);
}

/* Firefox scrollbar */
* {
  scrollbar-width: thin;
  scrollbar-color: rgba(128, 128, 128, 0.25) transparent;
}

html.dark * {
  scrollbar-color: rgba(255, 255, 255, 0.12) transparent;
}

/* ================================
   LAYOUT
   ================================ */
.app-container {
  display: flex;
  min-height: 100vh;
}

.sidebar {
  width: 300px;
  background-color: var(--color-surface);
  border-right: 1px solid var(--color-border);
  padding: var(--space-24);
  position: fixed;
  height: 100vh;
  overflow-y: auto;
  z-index: 100;
}

.logo {
  font-size: var(--font-size-xl);
  font-weight: var(--font-weight-bold);
  color: var(--color-primary);
  margin-bottom: var(--space-12);
  text-align: center;
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-8);
}

.logo .js-badge {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 28px;
  height: 28px;
  background: var(--color-js-yellow);
  color: #000;
  font-weight: 700;
  font-size: 13px;
  border-radius: var(--radius-sm);
  flex-shrink: 0;
}

/* Sidebar search */
.sidebar-search {
  position: relative;
  margin-bottom: var(--space-16);
}

.sidebar-search input {
  width: 100%;
  padding: var(--space-10) var(--space-12) var(--space-10) 36px;
  border-radius: var(--radius-base);
  border: 1px solid var(--color-border);
  background: var(--color-background);
  color: var(--color-text);
  font-size: var(--font-size-sm);
  font-family: var(--font-family-base);
  outline: none;
  transition: border-color var(--duration-fast) var(--ease-standard);
}

.sidebar-search input:focus {
  border-color: var(--color-primary);
  box-shadow: 0 0 0 3px var(--color-focus-ring);
}

.sidebar-search .search-icon {
  position: absolute;
  left: var(--space-12);
  top: 50%;
  transform: translateY(-50%);
  font-size: var(--font-size-sm);
  color: var(--color-text-secondary);
  pointer-events: none;
}

/* Navigation */
.nav-menu {
  list-style: none;
}

.nav-item {
  margin-bottom: var(--space-4);
}

.nav-item.hidden {
  display: none;
}

.nav-link {
  display: flex;
  align-items: center;
  padding: var(--space-10) var(--space-12);
  text-decoration: none;
  color: var(--color-text);
  border-radius: var(--radius-base);
  transition: all var(--duration-fast) var(--ease-standard);
  cursor: pointer;
  font-size: var(--font-size-sm);
  gap: var(--space-8);
}

.nav-link:hover {
  background-color: var(--color-secondary);
  color: var(--color-primary);
}

.nav-link.active {
  background-color: var(--color-primary);
  color: var(--color-btn-primary-text);
}

.nav-icon {
  font-size: 1em;
  flex-shrink: 0;
}

/* Main Content */
.main-content {
  flex: 1;
  margin-left: 300px;
  padding: var(--space-32);
  max-width: calc(100vw - 300px);
}

/* Sections */
.section {
  display: none;
  animation: fadeIn 0.5s ease-in-out;
}

.section.active {
  display: block;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(20px); }
  to { opacity: 1; transform: translateY(0); }
}

.section-header {
  margin-bottom: var(--space-32);
}

.section-title {
  font-size: var(--font-size-4xl);
  font-weight: var(--font-weight-bold);
  color: var(--color-primary);
  margin-bottom: var(--space-16);
}

.section-subtitle {
  font-size: var(--font-size-xl);
  color: var(--color-text-secondary);
  margin-bottom: var(--space-24);
}

/* ================================
   CONTENT CARDS
   ================================ */
.content-card {
  background-color: var(--color-surface);
  border-radius: var(--radius-lg);
  padding: var(--space-24);
  margin-bottom: var(--space-24);
  box-shadow: var(--shadow-sm);
}

.content-card h3 {
  font-size: var(--font-size-2xl);
  font-weight: var(--font-weight-bold);
  color: var(--color-primary);
  margin-bottom: var(--space-16);
}

.concept-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: var(--space-20);
  margin: var(--space-24) 0;
}

.concept-card {
  background: var(--color-bg-1);
  padding: var(--space-20);
  border-radius: var(--radius-md);
  border: 1px solid var(--color-border);
  transition: transform var(--duration-normal) var(--ease-standard), box-shadow var(--duration-normal) var(--ease-standard);
}

.concept-card:hover {
  transform: translateY(-4px);
  box-shadow: var(--shadow-md);
}

.concept-title {
  font-size: var(--font-size-lg);
  font-weight: var(--font-weight-semibold);
  color: var(--color-primary);
  margin-bottom: var(--space-12);
}

/* Highlight / Tip / Warning boxes */
.highlight-box {
  background-color: var(--color-bg-5);
  border-left: 4px solid var(--color-primary);
  padding: var(--space-16);
  margin: var(--space-20) 0;
  border-radius: var(--radius-base);
}

.highlight-box h4, .highlight-box strong {
  color: var(--color-primary);
}

.warning-box {
  background-color: var(--color-bg-6);
  border-left: 4px solid var(--color-warning);
  padding: var(--space-16);
  margin: var(--space-20) 0;
  border-radius: var(--radius-base);
}

.warning-box h4 {
  color: var(--color-warning);
}

.success-box {
  background-color: var(--color-bg-3);
  border-left: 4px solid var(--color-success);
  padding: var(--space-16);
  margin: var(--space-20) 0;
  border-radius: var(--radius-base);
}

.success-box h4 {
  color: var(--color-success);
}

/* ================================
   CODE BLOCKS (highlight.js)
   ================================ */
.code-block {
  position: relative;
  margin: var(--space-16) 0;
  border-radius: var(--radius-md);
  overflow: hidden;
  border: 1px solid var(--color-code-border);
  box-shadow: var(--shadow-sm);
}

.code-toolbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  background: var(--color-code-toolbar);
  padding: var(--space-6) var(--space-12);
  border-bottom: 1px solid var(--color-code-border);
}

.code-lang {
  font-family: var(--font-family-mono);
  font-size: var(--font-size-xs);
  color: var(--color-teal-400);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.copy-btn {
  display: inline-flex;
  align-items: center;
  gap: var(--space-4);
  padding: var(--space-4) var(--space-10);
  background: transparent;
  border: 1px solid rgba(255,255,255,0.15);
  border-radius: var(--radius-sm);
  color: rgba(255,255,255,0.7);
  font-size: var(--font-size-xs);
  font-family: var(--font-family-base);
  cursor: pointer;
  transition: all var(--duration-fast) var(--ease-standard);
}

.copy-btn:hover {
  background: rgba(255,255,255,0.08);
  color: #fff;
  border-color: rgba(255,255,255,0.3);
}

.copy-btn.copied {
  color: #a6e3a1;
  border-color: #a6e3a1;
}

.code-block pre {
  margin: 0;
  padding: var(--space-16);
  overflow-x: auto;
  background: var(--color-code-bg) !important;
}

.code-block pre code {
  font-family: var(--font-family-mono);
  font-size: 13px;
  line-height: 1.6;
  tab-size: 2;
  color: var(--color-code-text) !important;
}

/* Force highlight.js token colors to stay bright on dark bg */
.code-block pre code .hljs-keyword,
.code-block pre code .hljs-selector-tag { color: #cba6f7 !important; }
.code-block pre code .hljs-string,
.code-block pre code .hljs-template-variable { color: #a6e3a1 !important; }
.code-block pre code .hljs-comment { color: #6c7086 !important; }
.code-block pre code .hljs-number,
.code-block pre code .hljs-literal { color: #fab387 !important; }
.code-block pre code .hljs-built_in { color: #f38ba8 !important; }
.code-block pre code .hljs-title,
.code-block pre code .hljs-function { color: #89b4fa !important; }
.code-block pre code .hljs-variable,
.code-block pre code .hljs-attr { color: #f9e2af !important; }
.code-block pre code .hljs-params { color: #94e2d5 !important; }
.code-block pre code .hljs-property { color: #89dceb !important; }

/* Inline code */
p code, li code, td code, .inline-code {
  background: var(--color-secondary);
  padding: 2px 6px;
  border-radius: 4px;
  font-family: var(--font-family-mono);
  font-size: 0.9em;
  color: var(--color-primary);
  border: 1px solid var(--color-border);
}

/* ================================
   COMPARISON TABLE
   ================================ */
.example-table {
  width: 100%;
  border-collapse: collapse;
  margin: var(--space-20) 0;
}

.example-table th,
.example-table td {
  padding: var(--space-12);
  text-align: left;
  border-bottom: 1px solid var(--color-border);
}

.example-table th {
  background-color: var(--color-secondary);
  font-weight: var(--font-weight-semibold);
}

.example-table tr:hover {
  background-color: var(--color-secondary);
}

/* ================================
   COLLAPSIBLE DETAILS
   ================================ */
.collapsible {
  margin: var(--space-16) 0;
  border: 1px solid var(--color-border);
  border-radius: var(--radius-md);
  overflow: hidden;
}

.collapsible summary {
  padding: var(--space-16);
  cursor: pointer;
  background: var(--color-bg-8);
  font-weight: var(--font-weight-semibold);
  display: flex;
  align-items: center;
  gap: var(--space-8);
  transition: background var(--duration-fast) var(--ease-standard);
  list-style: none;
}

.collapsible summary::-webkit-details-marker {
  display: none;
}

.collapsible summary::before {
  content: '▶';
  font-size: var(--font-size-xs);
  transition: transform var(--duration-fast) var(--ease-standard);
  color: var(--color-primary);
}

.collapsible[open] summary::before {
  transform: rotate(90deg);
}

.collapsible summary:hover {
  background: var(--color-secondary-hover);
}

.collapsible .collapsible-content {
  padding: var(--space-16);
}

/* ================================
   MINI PLAYGROUND
   ================================ */
.playground {
  margin: var(--space-24) 0;
  border-radius: var(--radius-lg);
  border: 2px solid var(--color-primary);
  overflow: hidden;
  background: var(--color-surface);
  box-shadow: var(--shadow-md);
}

.playground-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: var(--space-12) var(--space-16);
  background: var(--color-primary);
  color: var(--color-btn-primary-text);
}

.playground-header h4 {
  font-size: var(--font-size-base);
  font-weight: var(--font-weight-semibold);
  display: flex;
  align-items: center;
  gap: var(--space-8);
}

.playground-run-btn {
  display: inline-flex;
  align-items: center;
  gap: var(--space-6);
  padding: var(--space-6) var(--space-16);
  background: var(--color-btn-primary-text);
  color: var(--color-primary);
  border: none;
  border-radius: var(--radius-sm);
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-semibold);
  font-family: var(--font-family-base);
  cursor: pointer;
  transition: all var(--duration-fast) var(--ease-standard);
}

.playground-run-btn:hover {
  transform: scale(1.05);
  box-shadow: var(--shadow-sm);
}

.playground-editor {
  width: 100%;
  min-height: 120px;
  padding: var(--space-16);
  background: var(--color-code-bg);
  color: var(--color-code-text);
  border: none;
  font-family: var(--font-family-mono);
  font-size: 13px;
  line-height: 1.6;
  resize: vertical;
  outline: none;
  tab-size: 2;
}

.playground-output {
  padding: var(--space-12) var(--space-16);
  background: var(--color-background);
  border-top: 1px solid var(--color-border);
  min-height: 40px;
  font-family: var(--font-family-mono);
  font-size: var(--font-size-sm);
  color: var(--color-text);
  white-space: pre-wrap;
}

.playground-output .output-label {
  font-size: var(--font-size-xs);
  color: var(--color-text-secondary);
  margin-bottom: var(--space-4);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.playground-output .output-line {
  padding: 2px 0;
}

.playground-output .output-error {
  color: var(--color-error);
}

/* ================================
   EVENT LOOP DIAGRAM
   ================================ */
.event-loop-container {
  background: var(--color-surface);
  border-radius: var(--radius-lg);
  padding: var(--space-32);
  margin: var(--space-24) 0;
  border: 1px solid var(--color-border);
}

.event-loop-diagram {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  gap: var(--space-24);
  margin: var(--space-24) 0;
}

.el-component {
  border-radius: var(--radius-md);
  padding: var(--space-20);
  text-align: center;
  border: 2px solid;
  position: relative;
}

.el-component h5 {
  font-size: var(--font-size-base);
  font-weight: var(--font-weight-bold);
  margin-bottom: var(--space-12);
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-6);
}

.el-stack {
  background: var(--color-bg-4);
  border-color: var(--color-red-400);
}

.el-stack h5 { color: var(--color-red-400); }

.el-webapi {
  background: var(--color-bg-2);
  border-color: var(--color-orange-400);
}

.el-webapi h5 { color: var(--color-orange-400); }

.el-queue {
  background: var(--color-bg-3);
  border-color: var(--color-success);
}

.el-queue h5 { color: var(--color-success); }

.el-item {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  padding: var(--space-8) var(--space-12);
  border-radius: var(--radius-sm);
  margin: var(--space-6) 0;
  font-family: var(--font-family-mono);
  font-size: var(--font-size-sm);
  animation: slideIn 0.3s var(--ease-standard);
}

@keyframes slideIn {
  from { opacity: 0; transform: translateX(-10px); }
  to { opacity: 1; transform: translateX(0); }
}

.el-loop-arrow {
  grid-column: 1 / -1;
  text-align: center;
  padding: var(--space-16);
  font-size: var(--font-size-lg);
  color: var(--color-primary);
  font-weight: var(--font-weight-bold);
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-12);
}

.el-loop-arrow .arrow-animation {
  display: inline-block;
  animation: pulse 1.5s ease-in-out infinite;
}

@keyframes pulse {
  0%, 100% { opacity: 0.4; transform: scale(0.95); }
  50% { opacity: 1; transform: scale(1.05); }
}

/* ================================
   STEPS LIST
   ================================ */
.steps-list {
  counter-reset: step-counter;
}

.step-item {
  counter-increment: step-counter;
  margin-bottom: var(--space-16);
  padding: var(--space-16);
  padding-left: 60px;
  background-color: var(--color-bg-7);
  border-radius: var(--radius-md);
  position: relative;
}

.step-item::before {
  content: counter(step-counter);
  position: absolute;
  left: var(--space-16);
  top: var(--space-16);
  background-color: var(--color-primary);
  color: var(--color-btn-primary-text);
  width: 30px;
  height: 30px;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  font-weight: var(--font-weight-bold);
  font-size: var(--font-size-sm);
}

/* ================================
   NAVIGATION BUTTONS
   ================================ */
.navigation {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-top: var(--space-32);
  padding-top: var(--space-24);
  border-top: 1px solid var(--color-border);
}

.btn {
  display: inline-flex;
  align-items: center;
  padding: var(--space-12) var(--space-20);
  background-color: var(--color-primary);
  color: var(--color-btn-primary-text);
  text-decoration: none;
  border-radius: var(--radius-base);
  font-weight: var(--font-weight-medium);
  border: none;
  cursor: pointer;
  transition: all var(--duration-fast) var(--ease-standard);
  font-family: var(--font-family-base);
}

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

.btn:disabled {
  opacity: 0.5;
  cursor: not-allowed;
}

.btn-secondary {
  background-color: var(--color-secondary);
  color: var(--color-text);
}

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

/* ================================
   RESOURCE GRID
   ================================ */
.resource-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(260px, 1fr));
  gap: var(--space-20);
  margin-top: var(--space-24);
}

.resource-card {
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-lg);
  padding: var(--space-24);
  text-align: center;
  transition: all 0.3s ease;
  position: relative;
  overflow: hidden;
}

.resource-card::before {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 4px;
  background: linear-gradient(90deg, var(--color-primary), var(--color-js-yellow));
  transform: scaleX(0);
  transition: transform 0.3s ease;
}

.resource-card:hover::before {
  transform: scaleX(1);
}

.resource-card:hover {
  transform: translateY(-6px);
  border-color: var(--color-primary);
  box-shadow: var(--shadow-lg);
}

.resource-icon {
  font-size: 2.5rem;
  margin-bottom: var(--space-12);
}

.resource-card h4 {
  color: var(--color-primary);
  margin-bottom: var(--space-8);
  font-size: var(--font-size-lg);
}

.resource-card p {
  color: var(--color-text-secondary);
  font-size: var(--font-size-sm);
  line-height: 1.5;
  margin-bottom: var(--space-16);
}

.resource-card a {
  color: var(--color-primary);
  text-decoration: none;
  font-weight: var(--font-weight-semibold);
  font-size: var(--font-size-sm);
  transition: color var(--duration-fast);
}

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

/* ================================
   PROGRESS BAR
   ================================ */
.progress-indicator {
  margin-bottom: var(--space-16);
  padding-bottom: var(--space-12);
  border-bottom: 1px solid var(--color-border);
}

.progress-bar-container {
  height: 6px;
  background: var(--color-secondary);
  border-radius: var(--radius-full);
  overflow: hidden;
  margin-bottom: var(--space-8);
}

.progress-bar-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--color-primary), var(--color-js-yellow));
  border-radius: var(--radius-full);
  width: 7.1%;
  transition: width 0.4s ease;
}

.progress-text {
  font-size: var(--font-size-xs);
  color: var(--color-text-secondary);
  display: flex;
  justify-content: center;
}

/* ================================
   HAMBURGER MENU
   ================================ */
.menu-toggle-btn {
  display: none;
  align-items: center;
  justify-content: center;
  gap: var(--space-8);
  background: var(--color-primary);
  border: none;
  color: var(--color-btn-primary-text);
  padding: var(--space-12) var(--space-16);
  border-radius: var(--radius-base);
  cursor: pointer;
  font-size: var(--font-size-base);
  font-weight: var(--font-weight-medium);
  width: 100%;
  margin-bottom: var(--space-12);
  transition: all 0.3s ease;
  font-family: var(--font-family-base);
}

.menu-toggle-btn:hover {
  background: var(--color-primary-hover);
}

.menu-toggle-btn.active {
  background: var(--color-primary-active);
}

.hamburger-icon {
  display: flex;
  flex-direction: column;
  gap: 4px;
  width: 20px;
}

.hamburger-icon span {
  display: block;
  height: 2px;
  background: currentColor;
  border-radius: 2px;
  transition: all 0.3s ease;
}

.menu-toggle-btn.active .hamburger-icon span:nth-child(1) {
  transform: rotate(45deg) translate(4px, 4px);
}

.menu-toggle-btn.active .hamburger-icon span:nth-child(2) {
  opacity: 0;
}

.menu-toggle-btn.active .hamburger-icon span:nth-child(3) {
  transform: rotate(-45deg) translate(4px, -4px);
}

.menu-text {
  font-family: var(--font-family-base);
}

/* ================================
   THEME TOGGLE
   ================================ */
.theme-toggle {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: var(--space-8);
  background: var(--color-secondary);
  border: 1px solid var(--color-border);
  color: var(--color-text);
  padding: var(--space-10) var(--space-16);
  border-radius: var(--radius-base);
  cursor: pointer;
  font-size: var(--font-size-sm);
  font-weight: var(--font-weight-medium);
  width: 100%;
  margin-top: var(--space-16);
  transition: all 0.3s ease;
  font-family: var(--font-family-base);
}

.theme-toggle:hover {
  background: var(--color-secondary-hover);
  border-color: var(--color-primary);
}

.theme-toggle-icon {
  font-size: 1.1em;
  transition: transform 0.3s ease;
}

html.dark .theme-toggle-icon.sun { display: inline; }
html.dark .theme-toggle-icon.moon { display: none; }
html:not(.dark) .theme-toggle-icon.sun { display: none; }
html:not(.dark) .theme-toggle-icon.moon { display: inline; }

/* ================================
   SCROLL TO TOP
   ================================ */
.scroll-to-top {
  position: fixed;
  bottom: 30px;
  right: 30px;
  width: 50px;
  height: 50px;
  background: var(--color-primary);
  color: var(--color-btn-primary-text);
  border: none;
  border-radius: 50%;
  cursor: pointer;
  font-size: 1.2rem;
  box-shadow: var(--shadow-md);
  opacity: 0;
  visibility: hidden;
  transition: all 0.3s ease;
  z-index: 1000;
  display: flex;
  align-items: center;
  justify-content: center;
}

.scroll-to-top.visible {
  opacity: 1;
  visibility: visible;
}

.scroll-to-top:hover {
  background: var(--color-primary-hover);
  transform: translateY(-3px);
  box-shadow: var(--shadow-lg);
}

/* ================================
   KEYBOARD HINT
   ================================ */
.keyboard-hint {
  position: fixed;
  bottom: 20px;
  left: 50%;
  transform: translateX(-50%);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  padding: 8px 16px;
  border-radius: var(--radius-base);
  font-size: var(--font-size-sm);
  color: var(--color-text-secondary);
  box-shadow: var(--shadow-sm);
  z-index: 100;
  display: flex;
  align-items: center;
  gap: 12px;
  opacity: 0.9;
  transition: opacity 0.5s;
}

.keyboard-hint kbd {
  background: var(--color-secondary);
  padding: 2px 8px;
  border-radius: 4px;
  font-family: var(--font-family-mono);
  font-size: 0.85em;
  border: 1px solid var(--color-border);
}

/* ================================
   FOOTER
   ================================ */
.site-footer {
  background: var(--color-surface);
  color: var(--color-text);
  padding: 40px 20px 24px;
  text-align: center;
  border-top: 3px solid var(--color-primary);
  margin-top: 40px;
  margin-left: 300px;
  width: calc(100% - 300px);
}

@media (max-width: 640px) {
  .site-footer {
    margin-left: 0;
    width: 100%;
    padding: 30px 16px 20px;
  }
}

.footer-content {
  max-width: 900px;
  margin: 0 auto;
  padding: 0 var(--space-16);
}

.footer-intro {
  font-size: clamp(0.95rem, 2vw, 1.1rem);
  margin-bottom: 16px;
  color: var(--color-text);
  line-height: 1.6;
}

.footer-intro .heart {
  color: #e74c3c;
  font-size: 1.3rem;
}

.footer-intro strong {
  color: var(--color-primary);
}

.footer-desc {
  font-size: clamp(0.85rem, 1.8vw, 0.95rem);
  color: var(--color-text-secondary);
  margin-bottom: 20px;
  line-height: 1.7;
}

.footer-author {
  margin-top: 24px;
  padding-top: 20px;
  border-top: 1px solid var(--color-border);
}

.footer-author p {
  font-size: 0.9rem;
  color: var(--color-text-secondary);
  margin-bottom: 12px;
}

.footer-author strong {
  color: var(--color-primary);
}

.github-link {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  background: var(--color-primary);
  color: var(--color-btn-primary-text);
  padding: 10px 20px;
  border-radius: 8px;
  text-decoration: none;
  font-size: 0.95rem;
  transition: all 0.3s ease;
}

.github-link:hover {
  background: var(--color-primary-hover);
  transform: translateY(-2px);
  box-shadow: var(--shadow-md);
}

.footer-copyright {
  font-size: 0.8rem;
  color: var(--color-text-secondary);
  margin-top: 20px;
}

/* Smooth theme transitions */
html {
  transition: background-color 0.3s ease, color 0.3s ease;
}

body, .sidebar, .content-card, .resource-card, .site-footer, .playground {
  transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease;
}

/* ================================
   RESPONSIVE
   ================================ */
@media (max-width: 1024px) {
  .sidebar { width: 260px; }
  .main-content {
    margin-left: 260px;
    padding: var(--space-24);
    max-width: calc(100vw - 260px);
  }
  .section-title { font-size: var(--font-size-3xl); }
  .event-loop-diagram { grid-template-columns: 1fr; }
}

@media (max-width: 768px) {
  .sidebar { width: 240px; padding: var(--space-16); }
  .main-content {
    margin-left: 240px;
    padding: var(--space-20);
    max-width: calc(100vw - 240px);
  }
  .section-title { font-size: var(--font-size-2xl); }
  .concept-grid { grid-template-columns: 1fr; }
  .event-loop-diagram { grid-template-columns: 1fr; }
  .resource-grid { grid-template-columns: 1fr; }
}

@media (max-width: 640px) {
  .app-container { flex-direction: column; }
  .sidebar {
    width: 100%;
    position: relative;
    height: auto;
    border-right: none;
    border-bottom: 1px solid var(--color-border);
    padding: var(--space-16);
  }
  #menu-toggle { display: flex !important; }
  .logo { font-size: var(--font-size-lg); margin-bottom: var(--space-8); }
  .nav-menu {
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.3s ease;
  }
  .nav-menu.active { max-height: 2000px; }
  .sidebar-search { margin-bottom: var(--space-8); max-height: 0; overflow: hidden; transition: max-height 0.3s ease; }
  .nav-menu.active ~ .sidebar-search, .sidebar-search.active { max-height: 60px; }
  .main-content {
    margin-left: 0;
    padding: var(--space-16);
    max-width: 100%;
    width: 100%;
  }
  .section-title { font-size: var(--font-size-xl); }
  .navigation { flex-direction: column; gap: var(--space-12); }
  .btn { width: 100%; justify-content: center; }
  .keyboard-hint { display: none; }
  .scroll-to-top { bottom: 20px; right: 20px; width: 45px; height: 45px; }
  .menu-toggle-btn { display: flex; }
  .event-loop-diagram { grid-template-columns: 1fr; }
  footer { padding: 30px 15px 20px 15px !important; }
}

@media (max-width: 480px) {
  .sidebar { padding: var(--space-12); }
  .main-content { padding: var(--space-12); }
  .section-title { font-size: var(--font-size-lg); }
  .content-card { padding: var(--space-12); }
  .concept-card { padding: var(--space-12); }
  .playground-editor { min-height: 80px; font-size: 12px; }
}