/* ===== CSS Custom Properties ===== */
:root {
  --font-sans: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen,
    Ubuntu, Cantarell, "Fira Sans", "Droid Sans", "Helvetica Neue", Arial,
    sans-serif;
  --font-mono: "SFMono-Regular", Consolas, "Liberation Mono", Menlo, monospace;

  --color-bg: #ffffff;
  --color-bg-secondary: #f7f8fa;
  --color-text: #1a1a2e;
  --color-text-secondary: #555770;
  --color-border: #e2e4e9;
  --color-accent: #2563eb;
  --color-accent-hover: #1d4ed8;
  --color-sidebar-bg: #f9fafb;
  --color-sidebar-active: #eff6ff;
  --color-sidebar-active-border: #2563eb;
  --color-slide-shadow: rgba(0, 0, 0, 0.08);
  --color-separator: #e5e7eb;
  --color-card-bg: #ffffff;
  --color-header-bg: #ffffff;

  --sidebar-width: 280px;
  --header-height: 56px;
  --content-max-width: 900px;
}

/* ===== Dark Mode ===== */
[data-theme="dark"] {
  --color-bg: #0f172a;
  --color-bg-secondary: #1e293b;
  --color-text: #e2e8f0;
  --color-text-secondary: #94a3b8;
  --color-border: #334155;
  --color-accent: #60a5fa;
  --color-accent-hover: #93bbfd;
  --color-sidebar-bg: #1e293b;
  --color-sidebar-active: #1e3a5f;
  --color-sidebar-active-border: #60a5fa;
  --color-slide-shadow: rgba(0, 0, 0, 0.3);
  --color-separator: #334155;
  --color-card-bg: #1e293b;
  --color-header-bg: #1e293b;
}

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

html {
  scroll-behavior: smooth;
  -webkit-text-size-adjust: 100%;
}

body {
  font-family: var(--font-sans);
  font-size: 16px;
  line-height: 1.6;
  color: var(--color-text);
  background-color: var(--color-bg);
  transition: background-color 0.2s, color 0.2s;
}

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

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

img {
  max-width: 100%;
  height: auto;
  display: block;
}

/* ===== Header ===== */
.site-header {
  position: fixed;
  top: 0;
  left: 0;
  right: 0;
  height: var(--header-height);
  background: var(--color-header-bg);
  border-bottom: 1px solid var(--color-border);
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 1.25rem;
  z-index: 100;
  transition: background-color 0.2s;
}

.site-header .site-title {
  font-size: 1.125rem;
  font-weight: 600;
  color: var(--color-text);
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.site-header .site-title a {
  color: inherit;
  text-decoration: none;
}

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

/* ===== Hamburger Button ===== */
.hamburger {
  display: none;
  background: none;
  border: none;
  cursor: pointer;
  padding: 0.5rem;
  color: var(--color-text);
}

.hamburger svg {
  width: 24px;
  height: 24px;
  fill: none;
  stroke: currentColor;
  stroke-width: 2;
  stroke-linecap: round;
  stroke-linejoin: round;
}

/* ===== Dark Mode Toggle ===== */
.dark-toggle {
  background: none;
  border: 1px solid var(--color-border);
  border-radius: 6px;
  cursor: pointer;
  padding: 0.375rem 0.5rem;
  color: var(--color-text);
  font-size: 1rem;
  line-height: 1;
  transition: border-color 0.2s;
}

.dark-toggle:hover {
  border-color: var(--color-accent);
}

/* ===== Layout ===== */
.layout {
  display: flex;
  margin-top: var(--header-height);
  min-height: calc(100vh - var(--header-height));
}

/* ===== Sidebar ===== */
.sidebar {
  position: fixed;
  top: var(--header-height);
  left: 0;
  bottom: 0;
  width: var(--sidebar-width);
  background: var(--color-sidebar-bg);
  border-right: 1px solid var(--color-border);
  overflow-y: auto;
  padding: 1rem 0;
  z-index: 90;
  transition: transform 0.25s ease, background-color 0.2s;
}

.sidebar-nav {
  list-style: none;
}

.sidebar-nav li {
  margin: 0;
}

.sidebar-nav .nav-lecture > a {
  display: block;
  padding: 0.5rem 1rem;
  font-size: 0.875rem;
  font-weight: 600;
  color: var(--color-text);
  border-left: 3px solid transparent;
  transition: background-color 0.15s, border-color 0.15s;
}

.sidebar-nav .nav-lecture > a:hover {
  background: var(--color-sidebar-active);
  text-decoration: none;
}

.sidebar-nav .nav-lecture.active > a {
  background: var(--color-sidebar-active);
  border-left-color: var(--color-sidebar-active-border);
  color: var(--color-accent);
}

.sidebar-nav .nav-sections {
  list-style: none;
  overflow: hidden;
  max-height: 0;
  transition: max-height 0.25s ease;
}

.sidebar-nav .nav-lecture.expanded .nav-sections {
  max-height: 1000px;
}

.sidebar-nav .nav-section a {
  display: block;
  padding: 0.35rem 1rem 0.35rem 1.75rem;
  font-size: 0.8125rem;
  color: var(--color-text-secondary);
  border-left: 3px solid transparent;
  transition: background-color 0.15s, color 0.15s;
}

.sidebar-nav .nav-section a:hover {
  background: var(--color-sidebar-active);
  color: var(--color-text);
  text-decoration: none;
}

.sidebar-nav .nav-section.active a {
  color: var(--color-accent);
  border-left-color: var(--color-sidebar-active-border);
}

/* ===== Main Content ===== */
.main-content {
  flex: 1;
  margin-left: var(--sidebar-width);
  padding: 2rem;
  max-width: calc(var(--content-max-width) + 4rem);
  transition: margin-left 0.25s ease;
}

/* ===== Lecture Header ===== */
.lecture-header {
  margin-bottom: 2.5rem;
  padding-bottom: 1.5rem;
  border-bottom: 2px solid var(--color-border);
}

.lecture-header h1 {
  font-size: 1.75rem;
  font-weight: 700;
  line-height: 1.3;
  color: var(--color-text);
}

.lecture-header .lecture-meta {
  margin-top: 0.5rem;
  font-size: 0.875rem;
  color: var(--color-text-secondary);
}

/* ===== Section Header ===== */
.section-header {
  margin: 2.5rem 0 1.5rem;
  padding-bottom: 0.75rem;
  border-bottom: 1px solid var(--color-separator);
}

.section-header h2 {
  font-size: 1.25rem;
  font-weight: 600;
  color: var(--color-text);
}

/* ===== Slide Block ===== */
.slide-block {
  margin-bottom: 3rem;
}

.slide-block .slide-image {
  display: block;
  width: 100%;
  max-width: var(--content-max-width);
  margin: 0 auto 1.25rem;
  border-radius: 6px;
  box-shadow: 0 2px 12px var(--color-slide-shadow);
  border: 1px solid var(--color-border);
}

.slide-block .slide-title {
  font-size: 1rem;
  font-weight: 600;
  margin-bottom: 0.5rem;
  color: var(--color-text);
}

.slide-block .slide-commentary {
  font-size: 0.9375rem;
  line-height: 1.7;
  color: var(--color-text-secondary);
  max-width: var(--content-max-width);
}

.slide-block .slide-commentary p {
  margin-bottom: 0.75rem;
}

.slide-block .slide-commentary p:last-child {
  margin-bottom: 0;
}

.slide-separator {
  border: none;
  border-top: 1px solid var(--color-separator);
  margin: 2rem 0;
}

/* ===== Index Page ===== */
.index-header {
  text-align: center;
  padding: 3rem 1rem 2rem;
}

.index-header h1 {
  font-size: 2rem;
  font-weight: 700;
  margin-bottom: 0.5rem;
}

.index-header p {
  font-size: 1.0625rem;
  color: var(--color-text-secondary);
  max-width: 600px;
  margin: 0 auto;
}

.lecture-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(300px, 1fr));
  gap: 1.25rem;
  max-width: 960px;
  margin: 0 auto;
  padding: 0 1rem 3rem;
}

.lecture-card {
  background: var(--color-card-bg);
  border: 1px solid var(--color-border);
  border-radius: 8px;
  padding: 1.25rem;
  transition: box-shadow 0.2s, border-color 0.2s;
}

.lecture-card:hover {
  border-color: var(--color-accent);
  box-shadow: 0 4px 16px var(--color-slide-shadow);
}

.lecture-card a {
  text-decoration: none;
  color: inherit;
  display: block;
}

.lecture-card .card-number {
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--color-accent);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  margin-bottom: 0.25rem;
}

.lecture-card .card-title {
  font-size: 1.0625rem;
  font-weight: 600;
  color: var(--color-text);
  margin-bottom: 0.375rem;
}

.lecture-card .card-meta {
  font-size: 0.8125rem;
  color: var(--color-text-secondary);
}

/* ===== Footer ===== */
.site-footer {
  padding: 1.5rem 2rem;
  text-align: center;
  font-size: 0.8125rem;
  color: var(--color-text-secondary);
  border-top: 1px solid var(--color-border);
  margin-left: var(--sidebar-width);
  transition: margin-left 0.25s ease;
}

/* Index page footer has no sidebar offset */
.index-page .site-footer {
  margin-left: 0;
}

/* ===== Sidebar Collapsed State ===== */
.sidebar-collapsed .sidebar {
  transform: translateX(calc(-1 * var(--sidebar-width)));
}

.sidebar-collapsed .main-content,
.sidebar-collapsed .site-footer {
  margin-left: 0;
}

/* ===== Responsive: Tablet & Mobile ===== */
@media (max-width: 768px) {
  .hamburger {
    display: block;
  }

  .sidebar {
    transform: translateX(calc(-1 * var(--sidebar-width)));
  }

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

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

  .main-content {
    margin-left: 0;
    padding: 1.25rem;
  }

  .site-footer {
    margin-left: 0;
  }

  .lecture-header h1 {
    font-size: 1.375rem;
  }

  .index-header h1 {
    font-size: 1.5rem;
  }

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

/* ===== Sidebar Overlay (mobile) ===== */
.sidebar-overlay {
  display: none;
  position: fixed;
  inset: 0;
  top: var(--header-height);
  background: rgba(0, 0, 0, 0.4);
  z-index: 80;
}