:root {
  --primary-blue: #2563eb;
  --bg-primary: #ffffff;
  --bg-secondary: #f8fafc;
  --text-primary: #0f172a;
  --text-secondary: #475569;
  --border-color: #e2e8f0;
}

[data-theme="dark"] {
  --bg-primary: #0f172a;
  --bg-secondary: #1e293b;
  --text-primary: #f8fafc;
  --text-secondary: #cbd5e1;
  --border-color: #334155;
}

body {
  font-family: 'Ubuntu', sans-serif;
  background-color: var(--bg-primary);
  color: var(--text-primary);
  transition: all 0.3s ease;
}

.navbar {
  background-color: var(--bg-primary) !important;
  border-bottom: 1px solid var(--border-color);
  box-shadow: 0 2px 4px rgba(0,0,0,0.1);
}

.navbar-nav .nav-link {
  color: var(--text-secondary) !important;
  font-weight: 500;
}

.navbar-nav .nav-link:hover,
.navbar-nav .nav-link.active {
  color: var(--primary-blue) !important;
}

.theme-toggle {
  background: var(--bg-secondary);
  border: 1px solid var(--border-color);
  border-radius: 20px;
  width: 50px;
  height: 26px;
  position: relative;
  cursor: pointer;
  margin-left: 1rem;
}

.theme-toggle-slider {
  position: absolute;
  top: 2px;
  left: 2px;
  width: 22px;
  height: 22px;
  background: var(--primary-blue);
  border-radius: 50%;
  transition: transform 0.3s ease;
  display: flex;
  align-items: center;
  justify-content: center;
  color: white;
  font-size: 12px;
}

[data-theme="dark"] .theme-toggle-slider {
  transform: translateX(24px);
}

.hero-section {
  background: linear-gradient(135deg, var(--primary-blue), #1d4ed8);
  color: white;
  padding: 4rem 0;
}

.course-section {
  background: var(--bg-secondary);
  border-radius: 12px;
  padding: 2rem;
  margin-bottom: 2rem;
  border: 1px solid var(--border-color);
}

.course-section h2 {
  color: var(--primary-blue);
  margin-bottom: 1.5rem;
  font-size: 1.5rem;
}

.lecture-grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
  gap: 1rem;
  margin-bottom: 1rem;
}

.unit-section {
  margin-bottom: 1.5rem;
  border: 1px solid var(--border-color);
  border-radius: 8px;
  overflow: hidden;
}

.unit-header {
  width: 100%;
  background: var(--bg-primary);
  border: none;
  padding: 1rem 1.25rem;
  text-align: left;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: space-between;
  transition: all 0.3s ease;
  font-size: 1rem;
  color: var(--text-primary);
}

.unit-header:hover {
  background: var(--bg-secondary);
}

.unit-header i.fa-chevron-right {
  transition: transform 0.3s ease;
  color: var(--primary-blue);
}

.unit-header.expanded i.fa-chevron-right {
  transform: rotate(90deg);
}

.lecture-count {
  font-size: 0.85rem;
  color: var(--text-secondary);
  font-weight: normal;
}

.unit-content {
  display: none;
  padding: 1rem;
  background: var(--bg-secondary);
}

.unit-content.expanded {
  display: grid;
}

.lecture-item {
  background: var(--bg-primary);
  border: 1px solid var(--border-color);
  border-radius: 8px;
  padding: 1rem;
  transition: all 0.3s ease;
}

.lecture-item:hover {
  transform: translateY(-2px);
  box-shadow: 0 4px 8px rgba(0,0,0,0.1);
}

.lecture-item h5 {
  color: var(--text-primary);
  font-size: 0.95rem;
  margin-bottom: 0.5rem;
  font-weight: 600;
}

.lecture-item p {
  color: var(--text-secondary);
  font-size: 0.85rem;
  margin-bottom: 0.75rem;
}

.btn-primary {
  background: var(--primary-blue);
  border: none;
  border-radius: 6px;
  padding: 0.4rem 0.8rem;
  font-size: 0.875rem;
  font-weight: 500;
  transition: all 0.3s ease;
}

.btn-primary:hover {
  background: #1d4ed8;
  transform: translateY(-1px);
}

.btn-outline-primary {
  color: var(--primary-blue);
  border: 1px solid var(--primary-blue);
  background: transparent;
  font-size: 0.875rem;
  padding: 0.4rem 0.8rem;
}

.btn-outline-primary:hover {
  background: var(--primary-blue);
  color: white;
}

.quick-links {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
  gap: 1rem;
  margin-bottom: 3rem;
}

.quick-link-card {
  background: var(--primary-blue);
  color: white;
  padding: 1.5rem;
  border-radius: 12px;
  text-decoration: none;
  transition: all 0.3s ease;
  display: block;
}

.quick-link-card:hover {
  background: #1d4ed8;
  transform: translateY(-4px);
  color: white;
  text-decoration: none;
}

.quick-link-card h4 {
  font-size: 1.25rem;
  margin-bottom: 0.5rem;
  font-weight: 600;
}

.quick-link-card p {
  font-size: 0.9rem;
  margin: 0;
  opacity: 0.9;
}

.quick-link-card small {
  font-size: 0.8rem;
  opacity: 0.85;
  display: block;
  margin-top: 0.5rem;
}

.footer {
  background: var(--bg-secondary);
  border-top: 1px solid var(--border-color);
  padding: 2rem 0;
  margin-top: 4rem;
  text-align: center;
  color: var(--text-secondary);
}

.footer-link {
  color: var(--text-secondary);
}

.footer-link:hover {
  color: var(--primary-blue);
}

@media (max-width: 768px) {
  .hero-section {
    padding: 2rem 0;
  }
  
  .course-section {
    padding: 1rem;
  }
  
  .lecture-grid,
  .quick-links {
    grid-template-columns: 1fr;
  }
  
  .quick-link-card {
    padding: 1rem;
  }
}