:root {
  --bg-primary: #0b1020;
  --bg-secondary: #121931;
  --bg-card: #18223f;
  --bg-card-hover: #1f2c52;
  --bg-surface: #0f162b;
  --text-primary: #f1f5f9;
  --text-secondary: #cbd5e1;
  --text-muted: #8b9bb4;
  --border-color: #293860;
  --tue-red: #d6002a;
  --tue-red-soft: rgba(214, 0, 42, 0.16);
  --accent-blue: #38bdf8;
  --accent-blue-soft: rgba(56, 189, 248, 0.15);
  --accent-indigo: #818cf8;
  --accent-emerald: #34d399;
  --accent-emerald-soft: rgba(52, 211, 153, 0.15);
  --accent-amber: #fbbf24;
  --accent-amber-soft: rgba(251, 191, 36, 0.15);
  --accent-rose: #fb7185;
  --accent-purple: #c084fc;
  --shadow-lg: 0 12px 32px -4px rgba(0, 0, 0, 0.45);
  --radius-md: 10px;
  --radius-lg: 14px;
  --font-sans: 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;
  --font-mono: 'JetBrains Mono', 'Fira Code', 'Consolas', monospace;
}

[data-theme="light"] {
  --bg-primary: #f8fafc;
  --bg-secondary: #ffffff;
  --bg-card: #ffffff;
  --bg-card-hover: #f1f5f9;
  --bg-surface: #f1f5f9;
  --text-primary: #0f172a;
  --text-secondary: #334155;
  --text-muted: #64748b;
  --border-color: #e2e8f0;
  --shadow-lg: 0 10px 25px -5px rgba(15, 23, 42, 0.08);
}

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

body {
  font-family: var(--font-sans);
  background-color: var(--bg-primary);
  color: var(--text-primary);
  line-height: 1.6;
  display: flex;
  min-height: 100vh;
  overflow-x: hidden;
}

/* Sidebar Navigation */
.sidebar {
  width: 330px;
  min-width: 330px;
  background-color: var(--bg-secondary);
  border-right: 1px solid var(--border-color);
  display: flex;
  flex-direction: column;
  height: 100vh;
  position: sticky;
  top: 0;
  z-index: 50;
}

.sidebar-header {
  padding: 18px 20px;
  border-bottom: 1px solid var(--border-color);
  background: linear-gradient(135deg, rgba(214, 0, 42, 0.14), rgba(56, 189, 248, 0.06));
}

.brand-badge {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.06em;
  color: #fff;
  background: var(--tue-red);
  padding: 3px 9px;
  border-radius: 999px;
  margin-bottom: 8px;
}

.sidebar-title {
  font-size: 1.15rem;
  font-weight: 800;
  letter-spacing: -0.02em;
  color: var(--text-primary);
}

.sidebar-subtitle {
  font-size: 0.8rem;
  color: var(--text-muted);
  margin-top: 2px;
}

.progress-box {
  padding: 14px 20px;
  border-bottom: 1px solid var(--border-color);
  background-color: var(--bg-surface);
}

.progress-header {
  display: flex;
  justify-content: space-between;
  font-size: 0.8rem;
  font-weight: 600;
  margin-bottom: 6px;
}

.progress-bar-track {
  width: 100%;
  height: 8px;
  background-color: var(--border-color);
  border-radius: 99px;
  overflow: hidden;
}

.progress-bar-fill {
  height: 100%;
  background: linear-gradient(90deg, var(--tue-red), var(--accent-blue), var(--accent-emerald));
  width: 0%;
  transition: width 0.35s ease;
}

.nav-scroll {
  flex: 1;
  overflow-y: auto;
  padding: 12px 12px 24px;
}

.nav-section-label {
  font-size: 0.72rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--text-muted);
  padding: 10px 10px 6px;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.nav-item {
  width: 100%;
  display: flex;
  align-items: flex-start;
  gap: 10px;
  padding: 10px 12px;
  border-radius: var(--radius-md);
  border: 1px solid transparent;
  background: transparent;
  color: var(--text-secondary);
  text-align: left;
  cursor: pointer;
  transition: all 0.15s ease;
  margin-bottom: 4px;
  font-family: inherit;
}

.nav-item:hover {
  background-color: var(--bg-card-hover);
  color: var(--text-primary);
}

.nav-item.active {
  background-color: var(--bg-card);
  border-color: var(--accent-blue);
  color: var(--text-primary);
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.2);
}

.lo-number {
  min-width: 28px;
  height: 28px;
  border-radius: 7px;
  background-color: var(--bg-surface);
  border: 1px solid var(--border-color);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 0.75rem;
  font-weight: 700;
  color: var(--accent-blue);
  flex-shrink: 0;
  margin-top: 1px;
}

.nav-item.active .lo-number {
  background-color: var(--accent-blue);
  color: #0b1020;
  border-color: var(--accent-blue);
}

.nav-item.completed .lo-number {
  background-color: var(--accent-emerald);
  color: #0b1020;
  border-color: var(--accent-emerald);
}

.nav-item-content {
  flex: 1;
  min-width: 0;
}

.nav-item-title {
  font-size: 0.84rem;
  font-weight: 600;
  line-height: 1.3;
  display: block;
}

.nav-item-meta {
  font-size: 0.72rem;
  color: var(--text-muted);
  margin-top: 2px;
  display: flex;
  align-items: center;
  gap: 6px;
}

/* Main Content Area */
.main-wrapper {
  flex: 1;
  display: flex;
  flex-direction: column;
  min-width: 0;
}

.topbar {
  height: 64px;
  border-bottom: 1px solid var(--border-color);
  background-color: var(--bg-secondary);
  padding: 0 28px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  position: sticky;
  top: 0;
  z-index: 40;
}

.topbar-left {
  display: flex;
  align-items: center;
  gap: 12px;
}

.topbar-actions {
  display: flex;
  align-items: center;
  gap: 10px;
}

.btn {
  display: inline-flex;
  align-items: center;
  gap: 7px;
  padding: 8px 14px;
  border-radius: 8px;
  font-size: 0.84rem;
  font-weight: 600;
  cursor: pointer;
  border: 1px solid var(--border-color);
  background-color: var(--bg-card);
  color: var(--text-primary);
  transition: all 0.15s ease;
  font-family: inherit;
  text-decoration: none;
}

.btn:hover {
  background-color: var(--bg-card-hover);
  border-color: var(--accent-blue);
}

.btn-primary {
  background-color: var(--accent-blue);
  color: #0b1020;
  border-color: var(--accent-blue);
}

.btn-primary:hover {
  background-color: #7dd3fc;
  border-color: #7dd3fc;
}

.btn-tue {
  background-color: var(--tue-red);
  color: #ffffff;
  border-color: var(--tue-red);
}

.btn-tue:hover {
  background-color: #f41c45;
}

.btn-success {
  background-color: var(--accent-emerald);
  color: #0b1020;
  border-color: var(--accent-emerald);
}

.content-container {
  max-width: 1240px;
  width: 100%;
  margin: 0 auto;
  padding: 28px;
}

/* Header Card for Learning Objective */
.lo-hero {
  background: linear-gradient(135deg, var(--bg-card), var(--bg-secondary));
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 24px 28px;
  margin-bottom: 22px;
  box-shadow: var(--shadow-lg);
}

.lo-badges {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-bottom: 12px;
}

.badge {
  display: inline-flex;
  align-items: center;
  gap: 5px;
  padding: 4px 10px;
  border-radius: 99px;
  font-size: 0.75rem;
  font-weight: 700;
}

.badge-lo {
  background-color: var(--tue-red-soft);
  color: #ff6b88;
  border: 1px solid rgba(214, 0, 42, 0.35);
}

.badge-week {
  background-color: var(--accent-blue-soft);
  color: var(--accent-blue);
  border: 1px solid rgba(56, 189, 248, 0.3);
}

.badge-reader {
  background-color: var(--accent-emerald-soft);
  color: var(--accent-emerald);
  border: 1px solid rgba(52, 211, 153, 0.3);
  text-decoration: none;
}

.badge-reader:hover {
  text-decoration: underline;
}

.lo-hero h1 {
  font-size: 1.55rem;
  font-weight: 800;
  line-height: 1.3;
  margin-bottom: 10px;
}

.lo-official-quote {
  font-size: 0.92rem;
  color: var(--text-secondary);
  background-color: var(--bg-surface);
  border-left: 4px solid var(--tue-red);
  padding: 10px 14px;
  border-radius: 0 8px 8px 0;
}

/* Mode Tabs inside a Learning Objective */
.mode-tabs {
  display: flex;
  gap: 8px;
  margin-bottom: 22px;
  border-bottom: 1px solid var(--border-color);
  padding-bottom: 12px;
  flex-wrap: wrap;
}

.mode-tab {
  padding: 10px 18px;
  border-radius: 9px;
  border: 1px solid var(--border-color);
  background-color: var(--bg-secondary);
  color: var(--text-secondary);
  font-weight: 600;
  font-size: 0.88rem;
  cursor: pointer;
  display: inline-flex;
  align-items: center;
  gap: 8px;
  transition: all 0.15s ease;
  font-family: inherit;
}

.mode-tab:hover {
  background-color: var(--bg-card-hover);
  color: var(--text-primary);
}

.mode-tab.active {
  background-color: var(--accent-blue);
  color: #0b1020;
  border-color: var(--accent-blue);
  font-weight: 700;
}

/* Cards & Grids */
.grid-2 {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(440px, 1fr));
  gap: 20px;
}

.grid-3 {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(300px, 1fr));
  gap: 18px;
}

.card {
  background-color: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 22px;
  margin-bottom: 20px;
  box-shadow: var(--shadow-lg);
}

.card h2 {
  font-size: 1.18rem;
  font-weight: 700;
  margin-bottom: 12px;
  color: var(--text-primary);
  display: flex;
  align-items: center;
  gap: 8px;
}

.card h3 {
  font-size: 1rem;
  font-weight: 700;
  margin: 14px 0 8px;
  color: var(--accent-blue);
}

.formula-box {
  background-color: var(--bg-surface);
  border: 1px solid var(--border-color);
  border-left: 4px solid var(--accent-blue);
  border-radius: 8px;
  padding: 14px 18px;
  margin: 12px 0;
  font-family: var(--font-mono);
  font-size: 0.92rem;
  overflow-x: auto;
  color: var(--text-primary);
}

.formula-label {
  font-family: var(--font-sans);
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  color: var(--accent-blue);
  margin-bottom: 4px;
  display: block;
}

.key-insight {
  background-color: var(--accent-amber-soft);
  border: 1px solid rgba(251, 191, 36, 0.35);
  border-radius: 8px;
  padding: 12px 16px;
  margin: 12px 0;
  font-size: 0.9rem;
}

/* Interactive Simulator Canvas & Controls */
.sim-container {
  background-color: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 22px;
  box-shadow: var(--shadow-lg);
}

.sim-layout {
  display: grid;
  grid-template-columns: 340px 1fr;
  gap: 22px;
  align-items: start;
}

@media (max-width: 1024px) {
  .sim-layout {
    grid-template-columns: 1fr;
  }
}

.sim-controls {
  background-color: var(--bg-surface);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  padding: 18px;
}

.control-group {
  margin-bottom: 14px;
}

.control-group label {
  display: flex;
  justify-content: space-between;
  font-size: 0.82rem;
  font-weight: 600;
  color: var(--text-secondary);
  margin-bottom: 5px;
}

.control-val {
  font-family: var(--font-mono);
  color: var(--accent-blue);
  font-weight: 700;
}

input[type="range"] {
  width: 100%;
  accent-color: var(--accent-blue);
  cursor: pointer;
}

select, input[type="text"], input[type="number"], textarea {
  width: 100%;
  padding: 8px 12px;
  background-color: var(--bg-card);
  border: 1px solid var(--border-color);
  border-radius: 6px;
  color: var(--text-primary);
  font-family: inherit;
  font-size: 0.88rem;
}

.sim-canvas-wrap {
  background-color: var(--bg-surface);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  padding: 14px;
  display: flex;
  flex-direction: column;
  gap: 12px;
}

canvas {
  width: 100%;
  background-color: #090d1a;
  border-radius: 8px;
  border: 1px solid var(--border-color);
  display: block;
}

.sim-readout {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
  gap: 10px;
}

.readout-item {
  background-color: var(--bg-card);
  border: 1px solid var(--border-color);
  padding: 10px 12px;
  border-radius: 8px;
}

.readout-label {
  font-size: 0.72rem;
  color: var(--text-muted);
  text-transform: uppercase;
  font-weight: 700;
}

.readout-value {
  font-family: var(--font-mono);
  font-size: 0.95rem;
  font-weight: 700;
  color: var(--accent-emerald);
  margin-top: 2px;
}

/* Worked Examples & Practice Quiz */
.example-step {
  border-left: 3px solid var(--accent-indigo);
  padding-left: 14px;
  margin: 10px 0;
}

.quiz-option {
  width: 100%;
  text-align: left;
  padding: 12px 16px;
  margin-bottom: 8px;
  border-radius: 8px;
  border: 1px solid var(--border-color);
  background-color: var(--bg-surface);
  color: var(--text-primary);
  cursor: pointer;
  font-size: 0.9rem;
  transition: all 0.15s ease;
  font-family: inherit;
}

.quiz-option:hover {
  border-color: var(--accent-blue);
  background-color: var(--bg-card-hover);
}

.quiz-option.correct {
  border-color: var(--accent-emerald);
  background-color: var(--accent-emerald-soft);
}

.quiz-option.wrong {
  border-color: var(--accent-rose);
  background-color: rgba(251, 113, 133, 0.15);
}

.quiz-explanation {
  margin-top: 12px;
  padding: 14px;
  border-radius: 8px;
  background-color: var(--bg-surface);
  border: 1px solid var(--border-color);
  font-size: 0.88rem;
}

/* Flashcards */
.flashcard-deck {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(290px, 1fr));
  gap: 16px;
}

.flashcard {
  background-color: var(--bg-surface);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  padding: 18px;
  cursor: pointer;
  min-height: 165px;
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  transition: transform 0.18s ease, border-color 0.18s ease;
}

.flashcard:hover {
  transform: translateY(-3px);
  border-color: var(--accent-blue);
}

.flashcard.flipped {
  background-color: var(--bg-card-hover);
  border-color: var(--accent-emerald);
}

/* Tables */
.data-table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.88rem;
  margin: 12px 0;
}

.data-table th, .data-table td {
  border: 1px solid var(--border-color);
  padding: 9px 12px;
  text-align: left;
}

.data-table th {
  background-color: var(--bg-surface);
  color: var(--accent-blue);
  font-weight: 700;
}

.lo-nav-footer {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-top: 28px;
  padding-top: 20px;
  border-top: 1px solid var(--border-color);
}