:root {
  --primary: #1e3a8a;
  --primary-hover: #1d4ed8;
  --primary-light: #eff6ff;
  --accent: #d97706;
  --accent-light: #fef3c7;
  --success: #059669;
  --success-light: #ecfdf5;
  --bg-main: #f8fafc;
  --bg-surface: #ffffff;
  --bg-sidebar: #0f172a;
  --text-main: #0f172a;
  --text-muted: #64748b;
  --border-color: #e2e8f0;
  --card-shadow: 0 4px 6px -1px rgb(0 0 0 / 0.07), 0 2px 4px -2px rgb(0 0 0 / 0.05);
  --card-hover-shadow: 0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.05);
  --radius-sm: 6px;
  --radius-md: 10px;
  --radius-lg: 16px;
  --font-sans: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  --font-serif: Georgia, Cambria, "Times New Roman", Times, serif;
}

[data-theme="dark"] {
  --primary: #60a5fa;
  --primary-hover: #93c5fd;
  --primary-light: #1e293b;
  --accent: #fbbf24;
  --accent-light: #451a03;
  --success: #34d399;
  --success-light: #064e3b;
  --bg-main: #0b0f19;
  --bg-surface: #131b2e;
  --bg-sidebar: #070a12;
  --text-main: #f1f5f9;
  --text-muted: #94a3b8;
  --border-color: #1e293b;
  --card-shadow: 0 4px 6px -1px rgb(0 0 0 / 0.4);
  --card-hover-shadow: 0 10px 15px -3px rgb(0 0 0 / 0.6);
}

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

body {
  font-family: var(--font-sans);
  background-color: var(--bg-main);
  color: var(--text-main);
  line-height: 1.65;
  font-size: 16px;
  transition: background-color 0.25s ease, color 0.25s ease;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

/* Reading Progress Bar */
#reading-progress {
  position: fixed;
  top: 0;
  left: 0;
  height: 4px;
  background: linear-gradient(90deg, #3b82f6, #10b981);
  width: 0%;
  z-index: 9999;
  transition: width 0.1s ease-out;
}

/* Header */
.site-header {
  background: var(--bg-surface);
  border-bottom: 1px solid var(--border-color);
  position: sticky;
  top: 0;
  z-index: 1000;
  box-shadow: 0 1px 3px 0 rgb(0 0 0 / 0.05);
}

.header-container {
  max-width: 1400px;
  margin: 0 auto;
  padding: 0.75rem 1.5rem;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
}

.brand-wrapper {
  display: flex;
  align-items: center;
  gap: 0.75rem;
  text-decoration: none;
  color: var(--text-main);
}

.brand-logo-icon {
  font-size: 1.8rem;
}

.brand-title {
  font-weight: 700;
  font-size: 1.1rem;
  line-height: 1.2;
}

.brand-subtitle {
  font-size: 0.75rem;
  color: var(--text-muted);
  text-transform: uppercase;
  letter-spacing: 0.05em;
}

.header-actions {
  display: flex;
  align-items: center;
  gap: 0.75rem;
}

.search-box {
  position: relative;
}

.search-box input {
  padding: 0.45rem 0.9rem 0.45rem 2rem;
  border-radius: var(--radius-sm);
  border: 1px solid var(--border-color);
  background: var(--bg-main);
  color: var(--text-main);
  font-size: 0.85rem;
  width: 220px;
  transition: all 0.2s ease;
}

.search-box input:focus {
  outline: none;
  border-color: var(--primary);
  width: 280px;
}

.search-icon {
  position: absolute;
  left: 0.6rem;
  top: 50%;
  transform: translateY(-50%);
  color: var(--text-muted);
  font-size: 0.85rem;
}

.theme-toggle-btn {
  background: var(--bg-main);
  border: 1px solid var(--border-color);
  color: var(--text-main);
  padding: 0.45rem 0.75rem;
  border-radius: var(--radius-sm);
  cursor: pointer;
  font-size: 0.9rem;
  display: flex;
  align-items: center;
  gap: 0.4rem;
  transition: all 0.2s ease;
}

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

/* Main Layout Grid */
.site-layout {
  max-width: 1400px;
  margin: 0 auto;
  padding: 2rem 1.5rem;
  display: grid;
  grid-template-columns: 300px 1fr;
  gap: 2.5rem;
  flex: 1;
  width: 100%;
}

@media (max-width: 980px) {
  .site-layout {
    grid-template-columns: 1fr;
    gap: 1.5rem;
  }
}

/* Sidebar Navigation */
.study-sidebar {
  position: sticky;
  top: 5rem;
  max-height: calc(100vh - 6rem);
  overflow-y: auto;
  background: var(--bg-surface);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  padding: 1.25rem;
  box-shadow: var(--card-shadow);
}

.sidebar-title {
  font-size: 0.85rem;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--text-muted);
  margin-bottom: 0.75rem;
  padding-bottom: 0.5rem;
  border-bottom: 1px solid var(--border-color);
}

.curriculum-list {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 0.35rem;
}

.curriculum-item a {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0.6rem 0.75rem;
  border-radius: var(--radius-sm);
  text-decoration: none;
  color: var(--text-main);
  font-size: 0.88rem;
  font-weight: 500;
  transition: all 0.15s ease;
}

.curriculum-item a:hover {
  background: var(--primary-light);
  color: var(--primary);
}

.curriculum-item.active a {
  background: var(--primary);
  color: #ffffff;
}

.curriculum-badge {
  font-size: 0.7rem;
  padding: 0.15rem 0.45rem;
  border-radius: 999px;
  background: var(--bg-main);
  color: var(--text-muted);
}

.curriculum-item.active .curriculum-badge {
  background: rgba(255, 255, 255, 0.25);
  color: #ffffff;
}

/* Main Content Area */
.content-card {
  background: var(--bg-surface);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-lg);
  padding: 2.5rem;
  box-shadow: var(--card-shadow);
}

@media (max-width: 640px) {
  .content-card {
    padding: 1.25rem;
  }
}

/* Module Header Card */
.module-header-card {
  margin-bottom: 2rem;
  padding-bottom: 1.5rem;
  border-bottom: 1px solid var(--border-color);
}

.module-badge-tag {
  display: inline-block;
  font-size: 0.75rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.05em;
  background: var(--primary-light);
  color: var(--primary);
  padding: 0.25rem 0.65rem;
  border-radius: var(--radius-sm);
  margin-bottom: 0.75rem;
}

.module-main-title {
  font-size: 2.2rem;
  font-weight: 800;
  line-height: 1.25;
  color: var(--text-main);
  margin-bottom: 0.5rem;
}

.module-subtitle {
  font-size: 1.1rem;
  color: var(--text-muted);
  margin-bottom: 1rem;
}

.module-meta {
  font-size: 0.85rem;
  color: var(--text-muted);
}

/* Sections */
.study-section {
  margin-bottom: 2.5rem;
}

.section-heading {
  font-size: 1.35rem;
  font-weight: 700;
  margin-bottom: 1rem;
  display: flex;
  align-items: center;
  gap: 0.5rem;
  color: var(--text-main);
}

.section-intro {
  font-size: 0.95rem;
  color: var(--text-muted);
  margin-bottom: 1rem;
}

/* Callout Cards */
.callout-card {
  padding: 1.25rem 1.5rem;
  border-radius: var(--radius-md);
  border-left: 4px solid var(--primary);
  background: var(--primary-light);
  font-size: 1.02rem;
  line-height: 1.6;
}

/* Takeaways */
.takeaways-list {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 0.75rem;
}

.takeaways-list li {
  position: relative;
  padding: 0.85rem 1rem 0.85rem 1.75rem;
  background: var(--bg-main);
  border-radius: var(--radius-md);
  border: 1px solid var(--border-color);
  font-size: 0.98rem;
}

.takeaways-list li::before {
  content: "✦";
  position: absolute;
  left: 0.75rem;
  top: 0.85rem;
  color: var(--primary);
}

/* Checklist Grid */
.checklist-grid {
  display: grid;
  grid-template-columns: 1fr;
  gap: 0.85rem;
}

.checklist-item {
  display: flex;
  gap: 1rem;
  align-items: flex-start;
  padding: 1rem;
  background: var(--bg-main);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  transition: all 0.15s ease;
}

.checklist-item:hover {
  border-color: var(--primary);
  transform: translateX(4px);
}

.checklist-num {
  background: var(--primary);
  color: #fff;
  font-weight: 700;
  font-size: 0.85rem;
  width: 26px;
  height: 26px;
  border-radius: 50%;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
  margin-top: 2px;
}

.checklist-content h4 {
  font-size: 1rem;
  font-weight: 700;
  margin-bottom: 0.25rem;
  color: var(--text-main);
}

.checklist-content p {
  font-size: 0.9rem;
  color: var(--text-muted);
  line-height: 1.5;
}

/* Glossary Cards */
.glossary-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 1rem;
}

.glossary-card {
  padding: 1rem 1.25rem;
  background: var(--bg-main);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
}

.glossary-term {
  display: block;
  font-weight: 700;
  font-size: 0.98rem;
  color: var(--primary);
  margin-bottom: 0.4rem;
}

.glossary-def {
  font-size: 0.88rem;
  color: var(--text-muted);
  line-height: 1.5;
}

/* Q&A Accordion */
.qa-accordion {
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

details.qa-item {
  background: var(--bg-main);
  border: 1px solid var(--border-color);
  border-radius: var(--radius-md);
  overflow: hidden;
  transition: all 0.2s ease;
}

details.qa-item[open] {
  border-color: var(--primary);
  box-shadow: var(--card-shadow);
  background: var(--bg-surface);
}

summary.qa-question {
  padding: 1.1rem 1.25rem;
  font-weight: 600;
  font-size: 1rem;
  color: var(--text-main);
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 1rem;
  user-select: none;
  list-style: none;
}

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

.qa-q-badge {
  background: var(--primary-light);
  color: var(--primary);
  font-weight: 700;
  font-size: 0.75rem;
  padding: 0.2rem 0.5rem;
  border-radius: var(--radius-sm);
  flex-shrink: 0;
}

.qa-q-text {
  flex: 1;
}

.qa-toggle-icon {
  font-size: 1.1rem;
  color: var(--text-muted);
  transition: transform 0.2s ease;
}

details.qa-item[open] .qa-toggle-icon {
  transform: rotate(180deg);
  color: var(--primary);
}

.qa-answer-box {
  padding: 1.25rem;
  border-top: 1px solid var(--border-color);
  background: var(--bg-surface);
  display: flex;
  flex-direction: column;
  gap: 1rem;
}

.qa-direct-answer {
  padding: 0.85rem 1rem;
  background: var(--success-light);
  border-radius: var(--radius-sm);
  font-size: 0.95rem;
}

.qa-direct-answer strong {
  color: var(--success);
  display: block;
  margin-bottom: 0.25rem;
}

.qa-explanation {
  padding: 0.85rem 1rem;
  background: var(--primary-light);
  border-radius: var(--radius-sm);
  font-size: 0.95rem;
}

.qa-explanation strong {
  color: var(--primary);
  display: block;
  margin-bottom: 0.25rem;
}

/* Footer */
.site-footer {
  background: var(--bg-surface);
  border-top: 1px solid var(--border-color);
  padding: 2rem 1.5rem;
  margin-top: auto;
  font-size: 0.88rem;
  color: var(--text-muted);
}

.footer-content {
  max-width: 1400px;
  margin: 0 auto;
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  gap: 1rem;
}