/* ============================================
   PMI Plus — Main Design System
   ============================================ */

/* --- CSS Custom Properties --- */
:root {
  /* Typography */
  --font-heading: Georgia, 'Times New Roman', serif;
  --font-body: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
  --font-mono: 'SF Mono', 'Fira Code', 'Fira Mono', Menlo, Consolas, monospace;

  /* Font sizes */
  --text-xs: 0.75rem;
  --text-sm: 0.875rem;
  --text-base: 1rem;
  --text-lg: 1.125rem;
  --text-xl: 1.25rem;
  --text-2xl: 1.5rem;
  --text-3xl: 1.875rem;
  --text-4xl: 2.25rem;
  --text-5xl: 3rem;

  /* Spacing scale */
  --sp-1: 0.25rem;
  --sp-2: 0.5rem;
  --sp-3: 0.75rem;
  --sp-4: 1rem;
  --sp-5: 1.25rem;
  --sp-6: 1.5rem;
  --sp-8: 2rem;
  --sp-10: 2.5rem;
  --sp-12: 3rem;
  --sp-16: 4rem;
  --sp-20: 5rem;

  /* Layout */
  --content-max-width: 900px;
  --sidebar-width: 240px;
  --nav-height: 56px;

  /* Transitions */
  --transition-fast: 150ms ease;
  --transition-base: 250ms ease;

  /* Border radius */
  --radius-sm: 4px;
  --radius-md: 8px;
  --radius-lg: 12px;
  --radius-xl: 16px;

  /* Chapter accent colors */
  --ch01-color: #4f46e5; /* indigo */
  --ch02-color: #0d9488; /* teal */
  --ch03-color: #d97706; /* amber */
  --ch04-color: #e11d48; /* rose */
  --ch05-color: #059669; /* emerald */
  --ch06-color: #0284c7; /* sky */
  --ch07-color: #7c3aed; /* violet */
  --ch08-color: #ea580c; /* orange */
  --ch09-color: #0891b2; /* cyan */
  --ch10-color: #db2777; /* pink */
  --ch11-color: #475569; /* slate */
  --ch12-color: #b45309; /* warm amber for appendix */

  /* Formula variable colors */
  --var-a: #e11d48;
  --var-b: #2563eb;
  --var-c: #059669;
  --var-d: #d97706;
  --var-e: #7c3aed;
  --var-f: #0891b2;
}

/* --- Light Theme (default) --- */
[data-theme="light"], :root {
  --bg-primary: #ffffff;
  --bg-secondary: #f8fafc;
  --bg-tertiary: #f1f5f9;
  --bg-code: #f1f5f9;
  --bg-nav: rgba(255, 255, 255, 0.92);
  --bg-card: #ffffff;
  --bg-card-hover: #f8fafc;
  --bg-formula: #fefce8;

  --text-primary: #0f172a;
  --text-secondary: #475569;
  --text-tertiary: #94a3b8;
  --text-link: #2563eb;
  --text-link-hover: #1d4ed8;

  --border-primary: #e2e8f0;
  --border-secondary: #cbd5e1;

  --shadow-sm: 0 1px 2px rgba(0,0,0,0.05);
  --shadow-md: 0 4px 6px -1px rgba(0,0,0,0.07), 0 2px 4px -2px rgba(0,0,0,0.05);
  --shadow-lg: 0 10px 15px -3px rgba(0,0,0,0.08), 0 4px 6px -4px rgba(0,0,0,0.04);
}

/* --- Dark Theme --- */
[data-theme="dark"] {
  --bg-primary: #0f172a;
  --bg-secondary: #1e293b;
  --bg-tertiary: #334155;
  --bg-code: #1e293b;
  --bg-nav: rgba(15, 23, 42, 0.92);
  --bg-card: #1e293b;
  --bg-card-hover: #334155;
  --bg-formula: #1a1a2e;

  --text-primary: #f1f5f9;
  --text-secondary: #94a3b8;
  --text-tertiary: #64748b;
  --text-link: #60a5fa;
  --text-link-hover: #93bbfd;

  --border-primary: #334155;
  --border-secondary: #475569;

  --shadow-sm: 0 1px 2px rgba(0,0,0,0.3);
  --shadow-md: 0 4px 6px -1px rgba(0,0,0,0.4), 0 2px 4px -2px rgba(0,0,0,0.3);
  --shadow-lg: 0 10px 15px -3px rgba(0,0,0,0.5), 0 4px 6px -4px rgba(0,0,0,0.3);
}

/* --- Reset & Base --- */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  font-size: 16px;
  scroll-behavior: smooth;
  scroll-padding-top: calc(var(--nav-height) + var(--sp-4));
}

body {
  font-family: var(--font-body);
  font-size: var(--text-base);
  line-height: 1.7;
  color: var(--text-primary);
  background-color: var(--bg-primary);
  transition: background-color var(--transition-base), color var(--transition-base);
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

/* --- Typography --- */
h1, h2, h3, h4, h5, h6 {
  font-family: var(--font-heading);
  line-height: 1.3;
  color: var(--text-primary);
  margin-bottom: var(--sp-4);
}

h1 {
  font-size: var(--text-4xl);
  letter-spacing: -0.02em;
  margin-bottom: var(--sp-6);
}

h2 {
  font-size: var(--text-2xl);
  margin-top: var(--sp-12);
  margin-bottom: var(--sp-4);
  padding-bottom: var(--sp-2);
  border-bottom: 2px solid var(--border-primary);
}

h3 {
  font-size: var(--text-xl);
  margin-top: var(--sp-8);
  margin-bottom: var(--sp-3);
}

h4 {
  font-size: var(--text-lg);
  margin-top: var(--sp-6);
  margin-bottom: var(--sp-2);
}

p {
  margin-bottom: var(--sp-4);
  color: var(--text-secondary);
}

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

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

strong {
  color: var(--text-primary);
  font-weight: 600;
}

code {
  font-family: var(--font-mono);
  font-size: 0.9em;
  background: var(--bg-code);
  padding: 0.15em 0.4em;
  border-radius: var(--radius-sm);
}

pre {
  background: var(--bg-code);
  border: 1px solid var(--border-primary);
  border-radius: var(--radius-md);
  padding: var(--sp-4);
  overflow-x: auto;
  margin-bottom: var(--sp-4);
}

pre code {
  background: none;
  padding: 0;
}

ul, ol {
  margin-bottom: var(--sp-4);
  padding-left: var(--sp-6);
  color: var(--text-secondary);
}

li {
  margin-bottom: var(--sp-2);
}

/* --- Top Navigation Bar --- */
.site-nav {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--nav-height);
  background: var(--bg-nav);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border-primary);
  z-index: 100;
  display: flex;
  align-items: center;
  padding: 0 var(--sp-6);
  transition: background-color var(--transition-base);
}

.nav-logo {
  font-family: var(--font-heading);
  font-size: var(--text-xl);
  font-weight: 700;
  color: var(--text-primary);
  text-decoration: none;
  display: flex;
  align-items: center;
  gap: var(--sp-2);
  flex-shrink: 0;
}

.nav-logo:hover {
  text-decoration: none;
  color: var(--text-primary);
}

.nav-logo .logo-accent {
  color: var(--ch01-color);
}

.nav-center {
  flex: 1;
  display: flex;
  justify-content: center;
  align-items: center;
}

.nav-chapter-select {
  font-family: var(--font-body);
  font-size: var(--text-sm);
  padding: var(--sp-1) var(--sp-3);
  border: 1px solid var(--border-primary);
  border-radius: var(--radius-md);
  background: var(--bg-secondary);
  color: var(--text-primary);
  cursor: pointer;
  max-width: 320px;
  width: 100%;
}

.nav-right {
  display: flex;
  align-items: center;
  gap: var(--sp-3);
  flex-shrink: 0;
}

.nav-progress {
  font-size: var(--text-xs);
  color: var(--text-tertiary);
  white-space: nowrap;
}

.theme-toggle {
  background: none;
  border: 1px solid var(--border-primary);
  border-radius: var(--radius-md);
  padding: var(--sp-1) var(--sp-2);
  cursor: pointer;
  color: var(--text-secondary);
  font-size: var(--text-lg);
  display: flex;
  align-items: center;
  justify-content: center;
  transition: all var(--transition-fast);
  width: 36px;
  height: 36px;
}

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

/* --- Page Layout --- */
.page-body {
  margin-top: var(--nav-height);
  display: flex;
  min-height: calc(100vh - var(--nav-height));
}

/* --- Sidebar --- */
.chapter-sidebar {
  width: var(--sidebar-width);
  flex-shrink: 0;
  padding: var(--sp-6) var(--sp-4);
  border-right: 1px solid var(--border-primary);
  position: sticky;
  top: var(--nav-height);
  height: calc(100vh - var(--nav-height));
  overflow-y: auto;
  background: var(--bg-secondary);
  transition: background-color var(--transition-base);
}

.sidebar-title {
  font-family: var(--font-heading);
  font-size: var(--text-sm);
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--text-tertiary);
  margin-bottom: var(--sp-4);
}

.sidebar-nav {
  list-style: none;
  padding: 0;
  margin: 0;
}

.sidebar-nav li {
  margin-bottom: var(--sp-1);
}

.sidebar-nav a {
  display: block;
  padding: var(--sp-1) var(--sp-3);
  font-size: var(--text-sm);
  color: var(--text-secondary);
  border-radius: var(--radius-sm);
  border-left: 2px solid transparent;
  transition: all var(--transition-fast);
}

.sidebar-nav a:hover {
  color: var(--text-primary);
  background: var(--bg-tertiary);
  text-decoration: none;
}

.sidebar-nav a.active {
  color: var(--text-link);
  background: var(--bg-tertiary);
  border-left-color: var(--text-link);
  font-weight: 500;
}

.sidebar-nav .sub-item a {
  padding-left: var(--sp-6);
  font-size: var(--text-xs);
}

/* --- Main Content --- */
.chapter-content {
  flex: 1;
  max-width: var(--content-max-width);
  margin: 0 auto;
  padding: var(--sp-8) var(--sp-6) var(--sp-16);
}

/* --- Chapter Header --- */
.chapter-header {
  margin-bottom: var(--sp-10);
}

.chapter-number {
  font-family: var(--font-mono);
  font-size: var(--text-sm);
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  margin-bottom: var(--sp-2);
}

.chapter-header h1 {
  margin-bottom: var(--sp-4);
}

.chapter-meta {
  display: flex;
  gap: var(--sp-4);
  align-items: center;
  flex-wrap: wrap;
  margin-bottom: var(--sp-6);
}

.chapter-meta span {
  font-size: var(--text-sm);
  color: var(--text-tertiary);
  display: flex;
  align-items: center;
  gap: var(--sp-1);
}

.what-youll-learn {
  background: var(--bg-secondary);
  border: 1px solid var(--border-primary);
  border-radius: var(--radius-lg);
  padding: var(--sp-5) var(--sp-6);
}

.what-youll-learn h4 {
  margin-top: 0;
  font-size: var(--text-base);
  color: var(--text-primary);
}

.what-youll-learn ul {
  margin-bottom: 0;
}

.what-youll-learn li {
  color: var(--text-secondary);
  font-size: var(--text-sm);
}

/* --- Chapter Footer Nav --- */
.chapter-footer-nav {
  display: flex;
  justify-content: space-between;
  gap: var(--sp-4);
  margin-top: var(--sp-16);
  padding-top: var(--sp-8);
  border-top: 2px solid var(--border-primary);
}

.chapter-nav-btn {
  display: flex;
  flex-direction: column;
  gap: var(--sp-1);
  padding: var(--sp-4) var(--sp-6);
  border: 1px solid var(--border-primary);
  border-radius: var(--radius-lg);
  text-decoration: none;
  transition: all var(--transition-fast);
  max-width: 45%;
}

.chapter-nav-btn:hover {
  background: var(--bg-secondary);
  border-color: var(--border-secondary);
  text-decoration: none;
}

.chapter-nav-btn .nav-label {
  font-size: var(--text-xs);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--text-tertiary);
}

.chapter-nav-btn .nav-title {
  font-family: var(--font-heading);
  font-size: var(--text-base);
  color: var(--text-primary);
}

.chapter-nav-btn.next {
  text-align: right;
  margin-left: auto;
}

/* --- Index Page --- */
.index-hero {
  text-align: center;
  padding: var(--sp-16) var(--sp-6) var(--sp-12);
  max-width: 720px;
  margin: 0 auto;
}

.index-hero h1 {
  font-size: var(--text-5xl);
  margin-bottom: var(--sp-4);
}

.index-hero .subtitle {
  font-size: var(--text-xl);
  color: var(--text-secondary);
  line-height: 1.6;
}

.index-hero .stats {
  display: flex;
  justify-content: center;
  gap: var(--sp-8);
  margin-top: var(--sp-8);
}

.index-hero .stat {
  text-align: center;
}

.index-hero .stat-number {
  font-family: var(--font-heading);
  font-size: var(--text-3xl);
  font-weight: 700;
  color: var(--text-primary);
  display: block;
}

.index-hero .stat-label {
  font-size: var(--text-sm);
  color: var(--text-tertiary);
}

/* --- Chapter Grid --- */
.chapter-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: var(--sp-6);
  max-width: 1100px;
  margin: 0 auto;
  padding: 0 var(--sp-6) var(--sp-16);
}

/* --- Progress Bar (overall) --- */
.overall-progress {
  max-width: 500px;
  margin: var(--sp-6) auto var(--sp-10);
  text-align: center;
}

.progress-bar-track {
  width: 100%;
  height: 6px;
  background: var(--bg-tertiary);
  border-radius: 3px;
  overflow: hidden;
  margin-top: var(--sp-2);
}

.progress-bar-fill {
  height: 100%;
  background: var(--ch01-color);
  border-radius: 3px;
  transition: width 0.5s ease;
}

.progress-label {
  font-size: var(--text-sm);
  color: var(--text-tertiary);
}

/* --- Mobile Sidebar Toggle --- */
.sidebar-toggle {
  display: none;
  background: none;
  border: 1px solid var(--border-primary);
  border-radius: var(--radius-md);
  padding: var(--sp-1) var(--sp-2);
  cursor: pointer;
  color: var(--text-secondary);
  font-size: var(--text-lg);
  width: 36px;
  height: 36px;
  align-items: center;
  justify-content: center;
}

/* --- Responsive --- */
@media (max-width: 768px) {
  :root {
    --sidebar-width: 260px;
  }

  .site-nav {
    padding: 0 var(--sp-3);
  }

  .nav-chapter-select {
    max-width: 180px;
  }

  .sidebar-toggle {
    display: flex;
  }

  .chapter-sidebar {
    position: fixed;
    left: calc(-1 * var(--sidebar-width));
    top: var(--nav-height);
    height: calc(100vh - var(--nav-height));
    z-index: 90;
    transition: left var(--transition-base);
    box-shadow: var(--shadow-lg);
  }

  .chapter-sidebar.open {
    left: 0;
  }

  .sidebar-overlay {
    display: none;
    position: fixed;
    inset: 0;
    top: var(--nav-height);
    background: rgba(0, 0, 0, 0.4);
    z-index: 89;
  }

  .sidebar-overlay.open {
    display: block;
  }

  .chapter-content {
    padding: var(--sp-4) var(--sp-4) var(--sp-12);
  }

  h1 {
    font-size: var(--text-3xl);
  }

  h2 {
    font-size: var(--text-xl);
  }

  .chapter-footer-nav {
    flex-direction: column;
  }

  .chapter-nav-btn {
    max-width: 100%;
  }

  .chapter-grid {
    grid-template-columns: 1fr;
    padding: 0 var(--sp-4) var(--sp-12);
  }

  .index-hero {
    padding: var(--sp-8) var(--sp-4) var(--sp-8);
  }

  .index-hero h1 {
    font-size: var(--text-3xl);
  }

  .index-hero .stats {
    gap: var(--sp-4);
  }
}

@media (max-width: 480px) {
  .nav-center {
    display: none;
  }

  .index-hero .stats {
    flex-direction: column;
    gap: var(--sp-2);
  }
}