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

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  line-height: 1.6;
  color: #333;
  background: #fff;
  margin: 0;
  padding: 0;
}

/* === Skip Navigation (Accessibility) === */
.skip-link {
  position: absolute;
  top: -100%;
  left: 1rem;
  background: #1B3A5C;
  color: #fff;
  padding: 0.5rem 1rem;
  z-index: 1000;
  border-radius: 0 0 4px 4px;
  text-decoration: none;
  font-weight: 600;
}
.skip-link:focus {
  top: 0;
}

/* === Header & Navigation === */
header[role="banner"] {
  background: #1B3A5C;
  color: #fff;
  padding: 0.75rem 1.5rem;
}
.nav-container {
  max-width: 960px;
  margin: 0 auto;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 1rem;
}
.nav-home {
  color: #fff;
  text-decoration: none;
  font-weight: 700;
  font-size: 1.1rem;
}
.nav-home:hover { text-decoration: underline; }
.nav-links {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-wrap: wrap;
  gap: 0.25rem;
}
.nav-links a {
  color: #D6EAF8;
  text-decoration: none;
  padding: 0.25rem 0.5rem;
  border-radius: 3px;
  font-size: 0.9rem;
}
.nav-links a:hover,
.nav-links a[aria-current="page"] {
  background: rgba(255,255,255,0.15);
  color: #fff;
  text-decoration: underline;
}

/* === Main Content === */
main {
  max-width: 960px;
  margin: 0 auto;
  padding: 1.5rem;
}

/* === Typography === */
h1 {
  color: #1B3A5C;
  font-size: 1.75rem;
  margin-top: 0;
  border-bottom: 2px solid #2E6DA4;
  padding-bottom: 0.5rem;
}
h2 {
  color: #2E6DA4;
  font-size: 1.35rem;
  margin-top: 2rem;
}
h3 {
  color: #1B3A5C;
  font-size: 1.1rem;
}
a { color: #2E6DA4; }
a:hover { color: #1B3A5C; }
a:focus {
  outline: 2px solid #2E6DA4;
  outline-offset: 2px;
}

/* === Breadcrumb === */
.breadcrumb {
  font-size: 0.85rem;
  color: #666;
  margin-bottom: 0.5rem;
}
.breadcrumb a { color: #2E6DA4; }

/* === Slide Count Badge === */
.slide-count {
  display: inline-block;
  background: #D6EAF8;
  color: #1B3A5C;
  padding: 0.2rem 0.6rem;
  border-radius: 3px;
  font-size: 0.85rem;
  font-weight: 600;
}

/* === Slide Sections === */
.lecture-content h2 {
  background: #f5f7fa;
  padding: 0.5rem 0.75rem;
  border-left: 4px solid #2E6DA4;
  margin-top: 2.5rem;
}
.lecture-content hr {
  border: none;
  border-top: 1px solid #ddd;
  margin: 2rem 0;
}

/* === Slide Images === */
.lecture-content img {
  max-width: 100%;
  height: auto;
  border: 1px solid #ddd;
  border-radius: 4px;
  margin: 1rem 0;
  display: block;
}

/* === Transcript Blocks === */
.lecture-content p strong:first-child {
  color: #1B3A5C;
}

/* === Key Concept Callouts === */
.key-concept {
  background: #f0f8e8;
  border-left: 4px solid #5a9e3a;
  padding: 0.75rem 1rem;
  margin: 1rem 0;
  border-radius: 0 4px 4px 0;
}

/* === Equation Display === */
.MathJax_Display {
  overflow-x: auto;
  padding: 0.5rem 0;
}

/* === Lecture Navigation === */
.lecture-nav {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-top: 3rem;
  padding-top: 1.5rem;
  border-top: 2px solid #ddd;
  gap: 1rem;
}
.lecture-nav a {
  padding: 0.5rem 1rem;
  background: #2E6DA4;
  color: #fff;
  text-decoration: none;
  border-radius: 4px;
  font-weight: 600;
}
.lecture-nav a:hover {
  background: #1B3A5C;
}
.lecture-nav .disabled { visibility: hidden; }
.lecture-nav .nav-up {
  background: transparent;
  color: #2E6DA4;
  border: 1px solid #2E6DA4;
}
.lecture-nav .nav-up:hover {
  background: #D6EAF8;
}

/* === Footer === */
footer {
  background: #f5f7fa;
  color: #595959;
  text-align: center;
  padding: 1.5rem;
  margin-top: 3rem;
  font-size: 0.85rem;
  border-top: 1px solid #ddd;
}
footer p { margin: 0.25rem 0; }
footer .license { font-size: 0.85rem; margin-top: 0.5rem; }
footer .license a { color: #2E6DA4; }

/* === Week Index Cards === */
.week-card {
  border: 1px solid #ddd;
  border-radius: 6px;
  padding: 1rem 1.25rem;
  margin: 1rem 0;
  transition: box-shadow 0.2s;
}
.week-card:hover {
  box-shadow: 0 2px 8px rgba(0,0,0,0.1);
}
.week-card h2 { margin-top: 0; border-left: none; background: none; padding: 0; }
.week-card ul {
  margin: 0.5rem 0 0 0;
  padding-left: 1.25rem;
}

/* === Responsive === */
@media (max-width: 640px) {
  main { padding: 1rem; }
  h1 { font-size: 1.4rem; }
  .lecture-nav { flex-direction: column; }
  .lecture-nav .disabled { display: none; }
  .nav-container { flex-direction: column; align-items: flex-start; }
}