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

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, sans-serif;
  line-height: 1.6;
  color: #333;
  background: #f8f9fa;
}

header {
  background: #fff;
  box-shadow: 0 2px 4px rgba(0,0,0,0.1);
  position: sticky;
  top: 0;
  z-index: 100;
}

.main-nav {
  max-width: 1200px;
  margin: 0 auto;
  padding: 1rem 2rem;
  display: flex;
  justify-content: space-between;
  align-items: center;
}

.logo {
  font-size: 1.5rem;
  font-weight: 700;
  color: #2563eb;
  text-decoration: none;
}

.nav-links {
  display: flex;
  list-style: none;
  gap: 2rem;
}

.nav-links a {
  color: #555;
  text-decoration: none;
  transition: color 0.3s;
}

.nav-links a:hover,
.nav-links a.active {
  color: #2563eb;
}

main {
  max-width: 900px;
  margin: 2rem auto;
  padding: 0 2rem;
}

.content {
  background: #fff;
  padding: 3rem;
  border-radius: 8px;
  box-shadow: 0 2px 8px rgba(0,0,0,0.05);
}

.content h1 {
  color: #1a1a1a;
  margin-bottom: 1.5rem;
  font-size: 2.5rem;
}

.content h2 {
  color: #333;
  margin: 2rem 0 1rem;
  font-size: 1.5rem;
}

.content p {
  margin-bottom: 1rem;
  color: #444;
}

.content ul, .content ol {
  margin: 1rem 0 1rem 2rem;
}

.content li {
  margin-bottom: 0.5rem;
}

.topics-overview {
  margin-top: 2rem;
  padding-top: 2rem;
  border-top: 1px solid #eee;
}

.topics-list {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
  gap: 1rem;
  list-style: none;
  margin: 1rem 0;
}

.topics-list li a {
  display: block;
  padding: 1rem;
  background: #f1f5f9;
  border-radius: 6px;
  color: #2563eb;
  text-decoration: none;
  transition: all 0.3s;
}

.topics-list li a:hover {
  background: #e2e8f0;
  transform: translateY(-2px);
}

.resources {
  margin-top: 2rem;
  padding-top: 2rem;
  border-top: 1px solid #eee;
}

.external-links {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem;
  margin-top: 1rem;
}

.external-links a {
  display: inline-block;
  padding: 0.5rem 1rem;
  background: #f8fafc;
  border: 1px solid #e2e8f0;
  border-radius: 4px;
  color: #475569;
  text-decoration: none;
  font-size: 0.9rem;
  transition: all 0.3s;
}

.external-links a:hover {
  background: #f1f5f9;
  border-color: #cbd5e1;
  color: #2563eb;
}

footer {
  background: #1a1a1a;
  color: #fff;
  margin-top: 3rem;
  padding: 3rem 2rem;
}

.footer-content {
  max-width: 1200px;
  margin: 0 auto;
  display: grid;
  grid-template-columns: 2fr 1fr;
  gap: 2rem;
}

.footer-nav h3 {
  margin-bottom: 1rem;
  color: #fff;
}

.footer-nav ul {
  list-style: none;
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
}

.footer-nav a {
  color: #94a3b8;
  text-decoration: none;
  transition: color 0.3s;
}

.footer-nav a:hover {
  color: #fff;
}

.footer-info {
  text-align: right;
  color: #64748b;
}

@media (max-width: 768px) {
  .main-nav {
    flex-direction: column;
    gap: 1rem;
  }

  .nav-links {
    flex-wrap: wrap;
    justify-content: center;
    gap: 1rem;
  }

  .content {
    padding: 1.5rem;
  }

  .content h1 {
    font-size: 1.75rem;
  }

  .footer-content {
    grid-template-columns: 1fr;
    text-align: center;
  }

  .footer-info {
    text-align: center;
  }
}