:root {
  /* Backgrounds */
  --bg: #fdfbf7;
  --bg-secondary: #f7f4ef;
  --bg-tertiary: #efeae2;
  --card-bg: #ffffff;

  /* Text */
  --text: #26231f;
  --text-secondary: #5c564d;
  --muted: #8f867b;

  /* Accents / borders */
  --border: #e6e0d6;
  --border-light: #f0ece4;
  --accent: #cf6b4b;
  --accent-hover: #a85438;
  --accent-soft: #f7e9e4;
  --accent-focus: rgba(207, 107, 75, 0.25);

  /* Elevation */
  --shadow: 0 4px 20px rgba(38, 35, 31, 0.06);
  --shadow-lg: 0 12px 40px rgba(38, 35, 31, 0.12);

  /* Layout */
  --sidebar-width: 280px;
  --top-nav-height: 60px;
  --max-content-width: 820px;

  /* Typography */
  --font-sans: Calibri, "Segoe UI", Roboto, "Helvetica Neue", "Noto Sans SC", sans-serif;
  --font-serif: Georgia, "Times New Roman", "Noto Serif SC", serif;
  --font-mono: "SFMono-Regular", Consolas, "Liberation Mono", Menlo, "Courier New", monospace;
}

* {
  box-sizing: border-box;
}

html {
  font-size: 16px;
}

body {
  margin: 0;
  font-family: var(--font-sans);
  background: var(--bg);
  color: var(--text);
  line-height: 1.7;
}

a {
  color: var(--accent);
  text-decoration: none;
}

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

/* Top navigation */
.top-nav {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--top-nav-height);
  background: rgba(253, 251, 247, 0.95);
  border-bottom: 1px solid var(--border);
  box-shadow: 0 1px 0 rgba(38, 35, 31, 0.04);
  backdrop-filter: blur(8px);
  z-index: 100;
}

.top-nav-inner {
  height: 100%;
  display: flex;
  align-items: center;
  padding: 0 1.25rem;
  gap: 1rem;
}

.top-nav-left {
  display: flex;
  align-items: center;
  gap: 1rem;
}

.site-brand {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  text-decoration: none;
}

.site-icon {
  width: 32px;
  height: 32px;
  border-radius: 8px;
  object-fit: cover;
  flex-shrink: 0;
}

.sidebar-toggle {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  width: 36px;
  height: 36px;
  padding: 0;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--card-bg);
  color: var(--text-secondary);
  cursor: pointer;
  transition: background 0.15s, border-color 0.15s, color 0.15s;
}

.sidebar-toggle:hover {
  background: var(--bg-secondary);
  border-color: var(--accent);
  color: var(--accent);
}

.sidebar-toggle:focus-visible {
  outline: none;
  box-shadow: 0 0 0 3px var(--accent-focus);
}

.sidebar-toggle .icon-close {
  display: none;
}

/* Sidebar is expanded (desktop not collapsed, or mobile drawer open) → show close icon */
.layout:not(.sidebar-collapsed) .sidebar-toggle .icon-menu,
.mobile-drawer-open .sidebar-toggle .icon-menu {
  display: none;
}

.layout:not(.sidebar-collapsed) .sidebar-toggle .icon-close,
.mobile-drawer-open .sidebar-toggle .icon-close {
  display: block;
}

.site-title {
  font-weight: 700;
  font-size: 1.15rem;
  color: var(--text);
  letter-spacing: -0.01em;
}

.site-brand:hover .site-title {
  color: var(--accent);
}

.site-brand:hover {
  text-decoration: none;
}

.top-links {
  display: flex;
  gap: 1.25rem;
  margin-left: 0.5rem;
}

.top-links a {
  color: var(--text-secondary);
  font-size: 0.95rem;
  font-weight: 500;
  padding: 0.25rem 0;
}

.top-links a:hover {
  color: var(--accent);
  text-decoration: none;
}

.search-box {
  margin-left: auto;
  position: relative;
}

#search-input {
  width: 260px;
  padding: 0.55rem 1rem;
  border: 1px solid var(--border);
  border-radius: 9999px;
  font-size: 0.9rem;
  background: var(--card-bg);
  color: var(--text);
  outline: none;
  transition: border-color 0.15s, box-shadow 0.15s;
}

#search-input:focus {
  border-color: var(--accent);
  box-shadow: 0 0 0 3px var(--accent-focus);
}

#search-input::placeholder {
  color: var(--muted);
}

.search-results {
  display: none;
  position: absolute;
  top: calc(100% + 10px);
  right: 0;
  width: 380px;
  max-height: 460px;
  overflow-y: auto;
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: 12px;
  box-shadow: var(--shadow-lg);
  padding: 0.5rem 0;
  z-index: 200;
}

.search-results.active {
  display: block;
}

.search-result-item {
  display: block;
  padding: 0.75rem 1rem;
  border-bottom: 1px solid var(--border-light);
}

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

.search-result-item:hover {
  background: var(--bg-secondary);
  text-decoration: none;
}

.search-result-title {
  font-weight: 600;
  font-size: 0.95rem;
  color: var(--text);
  margin-bottom: 0.2rem;
}

.search-result-snippet {
  font-size: 0.85rem;
  color: var(--muted);
  line-height: 1.5;
}

.search-result-meta {
  font-size: 0.8rem;
  color: var(--accent);
  margin-top: 0.25rem;
  font-weight: 500;
}

.search-no-results {
  padding: 1rem;
  color: var(--muted);
  font-size: 0.9rem;
}

mark.search-highlight {
  background: #fde68a;
  color: inherit;
  padding: 0.1em 0.2em;
  border-radius: 3px;
}

.search-result-snippet mark {
  background: transparent;
  color: var(--accent);
  font-weight: 600;
  padding: 0;
}

/* Layout */
.layout {
  display: grid;
  grid-template-columns: var(--sidebar-width) 1fr;
  transition: grid-template-columns 0.25s ease;
  padding-top: var(--top-nav-height);
  min-height: 100vh;
}

.layout.sidebar-collapsed {
  grid-template-columns: 0px 1fr;
}

.sidebar {
  position: sticky;
  top: var(--top-nav-height);
  height: calc(100vh - var(--top-nav-height));
  overflow: hidden;
  background: var(--bg-secondary);
  border-right: 1px solid var(--border);
}

.sidebar-inner {
  width: var(--sidebar-width);
  height: 100%;
  overflow-y: auto;
  padding: 1.25rem 0;
}

.sidebar-backdrop {
  display: none;
}

.sidebar-section {
  margin-bottom: 1.25rem;
  padding: 0 1rem;
}

.sidebar-heading {
  font-size: 0.72rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: 0.08em;
  color: var(--muted);
  margin-bottom: 0.5rem;
  padding: 0 0.75rem;
}

.sidebar-link {
  display: block;
  padding: 0.55rem 0.75rem;
  border-radius: 8px;
  color: var(--text-secondary);
  font-size: 0.92rem;
  margin-bottom: 0.15rem;
  transition: background 0.15s, color 0.15s;
}

.sidebar-link:hover {
  background: var(--border-light);
  color: var(--text);
  text-decoration: none;
}

.sidebar-link.active {
  background: var(--accent-soft);
  color: var(--accent-hover);
  font-weight: 600;
}

.sidebar-link.nested {
  padding-left: 1.25rem;
  font-size: 0.88rem;
}

.course-sections {
  margin-top: 0.25rem;
  margin-bottom: 0.5rem;
}

.section-group {
  margin-bottom: 0.75rem;
}

.section-title {
  font-size: 0.75rem;
  font-weight: 700;
  color: var(--muted);
  padding: 0.35rem 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.06em;
}

/* Main content */
.main {
  margin-left: 0;
  flex: 1;
  padding: 2.5rem;
  min-width: 0;
}

.content {
  max-width: var(--max-content-width);
  margin: 0 auto;
}

.breadcrumbs {
  font-size: 0.85rem;
  color: var(--muted);
  margin-bottom: 1.5rem;
}

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

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

.breadcrumbs .sep {
  margin: 0 0.4rem;
  color: var(--border);
}

.content h1,
.content h2,
.content h3,
.content h4 {
  font-family: var(--font-serif);
  font-weight: 600;
  line-height: 1.25;
  margin-top: 2rem;
  margin-bottom: 1rem;
  color: var(--text);
  letter-spacing: -0.01em;
}

.content h1 {
  font-size: 2.25rem;
  margin-top: 0;
  padding-bottom: 0.75rem;
  border-bottom: 1px solid var(--border);
}

.content h2 {
  font-size: 1.6rem;
  border-bottom: 1px solid var(--border);
  padding-bottom: 0.4rem;
}

.content h3 {
  font-size: 1.3rem;
}

.content h4 {
  font-size: 1.1rem;
}

.content p {
  margin: 0 0 1rem;
}

.content ul,
.content ol {
  margin: 0 0 1rem 1.5rem;
  padding: 0;
}

.content li {
  margin-bottom: 0.4rem;
}

.content blockquote {
  margin: 1.5rem 0;
  padding: 1rem 1.25rem;
  border-left: 4px solid var(--accent);
  border-radius: 0 10px 10px 0;
  background: var(--bg-secondary);
  color: var(--text-secondary);
}

.content blockquote p:last-child {
  margin-bottom: 0;
}

/* Tables */
.table-wrapper {
  overflow-x: auto;
  margin: 1.5rem 0;
  border: 1px solid var(--border);
  border-radius: 10px;
  background: var(--card-bg);
  box-shadow: var(--shadow);
}

.content table {
  width: 100%;
  border-collapse: collapse;
  font-size: 0.92rem;
}

.content th,
.content td {
  padding: 0.7rem 0.9rem;
  border-bottom: 1px solid var(--border-light);
  text-align: left;
}

.content th {
  background: var(--bg-secondary);
  font-weight: 600;
  color: var(--text);
}

.content tr:last-child td {
  border-bottom: none;
}

.content tr:hover td {
  background: var(--bg-secondary);
}

/* Code */
.content pre {
  background: var(--bg-tertiary);
  border: 1px solid var(--border);
  border-radius: 10px;
  padding: 1rem;
  overflow-x: auto;
  font-family: var(--font-mono);
  font-size: 0.88rem;
  line-height: 1.55;
  margin: 1.25rem 0;
}

.content code {
  font-family: var(--font-mono);
  font-size: 0.9em;
}

.content p code,
.content li code {
  background: var(--bg-tertiary);
  padding: 0.15rem 0.35rem;
  border-radius: 5px;
}

/* Images */
.content img {
  max-width: 100%;
  height: auto;
  border-radius: 10px;
  margin: 1rem 0;
}

/* Prev/Next navigation */
.prev-next {
  display: flex;
  justify-content: space-between;
  margin-top: 3rem;
  padding-top: 1.5rem;
  border-top: 1px solid var(--border);
}

.prev-next .prev,
.prev-next .next {
  flex: 1;
}

.prev-next .next {
  text-align: right;
}

/* Homepage course grid */
.course-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(260px, 1fr));
  gap: 1.25rem;
  margin-top: 1.5rem;
}

.course-card {
  display: block;
  padding: 1.5rem;
  border: 1px solid var(--border);
  border-radius: 12px;
  background: var(--card-bg);
  box-shadow: var(--shadow);
  transition: box-shadow 0.2s, transform 0.2s, border-color 0.2s;
}

.course-card:hover {
  box-shadow: var(--shadow-lg);
  transform: translateY(-2px);
  border-color: var(--accent);
  text-decoration: none;
}

.course-card h3 {
  margin: 0 0 0.5rem;
  font-size: 1.2rem;
  color: var(--text);
  font-family: var(--font-serif);
}

.course-card p {
  margin: 0;
  color: var(--muted);
  font-size: 0.9rem;
}

/* KaTeX adjustments */
.katex-display {
  margin: 1.25rem 0;
  overflow-x: auto;
  overflow-y: hidden;
}

/* Responsive */
@media (max-width: 900px) {
  .layout {
    display: block;
  }

  .sidebar {
    position: fixed;
    top: var(--top-nav-height);
    left: 0;
    width: min(280px, 85vw);
    height: calc(100vh - var(--top-nav-height));
    transform: translateX(-100%);
    transition: transform 0.25s ease;
    z-index: 150;
    border-right: 1px solid var(--border);
  }

  .sidebar.mobile-open {
    transform: translateX(0);
  }

  .sidebar-backdrop {
    display: block;
    position: fixed;
    inset: var(--top-nav-height) 0 0 0;
    background: rgba(38, 35, 31, 0.35);
    opacity: 0;
    visibility: hidden;
    transition: opacity 0.25s ease, visibility 0.25s;
    z-index: 140;
  }

  .sidebar-backdrop.active {
    opacity: 1;
    visibility: visible;
  }

  .main {
    padding: 1.25rem;
  }

  .search-box {
    display: none;
  }

  .top-links {
    display: none;
  }
}

@media (max-width: 600px) {
  .top-nav-inner {
    padding: 0 1rem;
  }

  .site-title {
    font-size: 1.05rem;
  }

  .content h1 {
    font-size: 1.8rem;
  }

  .content h2 {
    font-size: 1.35rem;
  }

  .content h3 {
    font-size: 1.15rem;
  }

  .course-grid {
    grid-template-columns: 1fr;
  }
}

@media (prefers-reduced-motion: reduce) {
  .layout,
  .sidebar,
  .sidebar-backdrop,
  .course-card,
  .sidebar-toggle,
  .sidebar-link,
  #search-input {
    transition: none !important;
  }
}