/* ===== CSS Variables ===== */
:root {
  /* Light theme variables */
  --bg: #f5f5f7;
  --bg-alt: #ffffff;
  --card: #ffffff;
  --accent: #9333ea;
  --accent-hover: #7e22ce;
  --accent-soft: rgba(147, 51, 234, 0.06);
  --accent-glow: rgba(147, 51, 234, 0.25);
  --accent-link: #6b7280;
  --text: #1f2937;
  --text-muted: #6b7280;
  --border: #e5e7eb;
  --shadow: 0 10px 30px rgba(0, 0, 0, 0.08);
  --gradient-start: #fafafa;
  --gradient-mid: #f5f5f7;
  --gradient-end: #e5e7eb;
  --radius-lg: 1.25rem;
}

[data-theme="dark"] {
  --bg: #111827;
  --bg-alt: #1f2937;
  --card: #1f2937;
  --accent: #a855f7;
  --accent-hover: #9333ea;
  --accent-soft: rgba(168, 85, 247, 0.1);
  --accent-glow: rgba(168, 85, 247, 0.3);
  --accent-link: #9ca3af;
  --text: #f9fafb;
  --text-muted: #9ca3af;
  --border: #374151;
  --shadow: 0 10px 30px rgba(0, 0, 0, 0.5);
  --gradient-start: #0f172a;
  --gradient-mid: #111827;
  --gradient-end: #1f2937;
}

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

body {
  font-family: 'Inter', system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  background: linear-gradient(
    to bottom,
    var(--gradient-start) 0%,
    var(--gradient-mid) 50%,
    var(--gradient-end) 100%
  );
  color: var(--text);
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 2rem 1rem;
  transition: background 0.3s ease, color 0.3s ease;
}

.shell {
  width: 100%;
  max-width: 920px;
  border-radius: 1.5rem;
  background: var(--bg-alt);
  border: 1px solid var(--border);
  box-shadow: var(--shadow);
  overflow: hidden;
  position: relative;
}

.shell::before {
  content: "";
  position: absolute;
  top: 0;
  right: 0;
  width: 300px;
  height: 300px;
  background: radial-gradient(
    circle,
    var(--accent-soft),
    transparent 70%
  );
  pointer-events: none;
}

/* Theme Toggle Button */
.theme-toggle {
  position: fixed;
  top: 1.5rem;
  right: 1.5rem;
  width: 3rem;
  height: 3rem;
  border-radius: 50%;
  background: var(--card);
  border: 2px solid var(--border);
  display: flex;
  align-items: center;
  justify-content: center;
  cursor: pointer;
  transition: all 0.3s ease;
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
  z-index: 1000;
}

.theme-toggle:hover {
  transform: scale(1.1);
  box-shadow: 0 6px 20px rgba(0, 0, 0, 0.15);
  border-color: var(--accent);
}

.theme-toggle svg {
  width: 1.25rem;
  height: 1.25rem;
  fill: var(--text);
  transition: fill 0.3s ease;
}

header {
  padding: 2rem 2rem 1.5rem;
  border-bottom: 1px solid var(--border);
  position: relative;
  z-index: 1;
}

.title-main {
  font-size: clamp(1.5rem, 3vw + 0.5rem, 2rem);
  font-weight: 700;
  letter-spacing: -0.02em;
  color: var(--text);
  display: flex;
  justify-content: space-between;
  align-items: baseline;
  gap: 1rem;
  margin-bottom: 0.5rem;
  flex-wrap: wrap;
}

.title-main span:first-child {
  background: linear-gradient(135deg, var(--accent) 0%, var(--accent-hover) 100%);
  -webkit-background-clip: text;
  -webkit-text-fill-color: transparent;
  background-clip: text;
}

.title-main span:last-child {
  font-size: 0.8rem;
  font-weight: 500;
  color: var(--text-muted);
}

.subtitle-main {
  font-size: 0.95rem;
  color: var(--text-muted);
  line-height: 1.5;
}

main {
  padding: 1.5rem 2rem 2rem;
  position: relative;
  z-index: 1;
}

.grid {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  gap: 1.25rem;
  margin-bottom: 1.5rem;
}

.card {
  border-radius: var(--radius-lg);
  border: 1px solid var(--border);
  background: var(--card);
  padding: 1.5rem;
  position: relative;
  overflow: hidden;
  transition: all 0.3s ease;
}

.card::before {
  content: "";
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
  height: 3px;
  background: linear-gradient(90deg, var(--accent), var(--accent-hover));
  transform: scaleX(0);
  transform-origin: left;
  transition: transform 0.3s ease;
}

.card:hover {
  transform: translateY(-4px);
  box-shadow: 0 12px 40px rgba(0, 0, 0, 0.12);
  border-color: var(--accent);
}

.card:hover::before {
  transform: scaleX(1);
}

.card-header {
  display: flex;
  justify-content: space-between;
  align-items: flex-start;
  gap: 0.75rem;
  margin-bottom: 0.75rem;
}

.card-title {
  font-size: 1.1rem;
  font-weight: 600;
  color: var(--text);
}

.card-tag {
  font-size: 0.7rem;
  padding: 0.25rem 0.65rem;
  border-radius: 999px;
  background: var(--accent-soft);
  border: 1px solid var(--accent);
  color: var(--accent);
  text-transform: uppercase;
  letter-spacing: 0.05em;
  font-weight: 600;
  white-space: nowrap;
  flex-shrink: 0;
}

.card-body {
  font-size: 0.875rem;
  color: var(--text-muted);
  margin-bottom: 0.75rem;
  line-height: 1.6;
}

.card-meta {
  display: flex;
  justify-content: space-between;
  align-items: center;
  font-size: 0.75rem;
  color: var(--text-muted);
  gap: 0.75rem;
  flex-wrap: wrap;
  margin-top: 1rem;
}

.card-meta code {
  background: var(--bg);
  padding: 0.2rem 0.5rem;
  border-radius: 0.3rem;
  font-size: 0.7rem;
}

.card-link {
  display: inline-flex;
  align-items: center;
  gap: 0.4rem;
  font-size: 0.8rem;
  font-weight: 600;
  color: var(--text);
  text-decoration: none;
  border-radius: 0.5rem;
  padding: 0.5rem 1rem;
  background: var(--bg);
  border: 1px solid var(--border);
  transition: all 0.2s ease;
}

.card-link:hover {
  background: var(--accent);
  color: white;
  border-color: var(--accent);
  transform: translateY(-2px);
  box-shadow: 0 4px 12px var(--accent-glow);
}

.card-link svg {
  width: 14px;
  height: 14px;
}

.course-link {
  color: var(--accent-link);
  text-decoration: none;
  font-weight: 500;
  transition: all 0.2s ease;
  border-bottom: 1px solid transparent;
}

.course-link:hover {
  color: var(--accent);
  border-bottom-color: var(--accent);
}

.footer {
  font-size: 0.75rem;
  color: var(--text-muted);
  text-align: center;
  border-top: 1px solid var(--border);
  padding-top: 1rem;
  margin-top: 1rem;
}

@media (max-width: 768px) {
  .title-main {
    flex-direction: column;
    align-items: flex-start;
  }
  
  .title-main span:last-child {
    font-size: 0.75rem;
  }
}

@media (max-width: 640px) {
  header {
    padding: 1.5rem 1.5rem 1rem;
  }

  main {
    padding: 1.25rem 1.5rem 1.5rem;
  }

  .theme-toggle {
    top: 1rem;
    right: 1rem;
    width: 2.75rem;
    height: 2.75rem;
  }

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