:root {
  --primary: #6c5ce7;
  --primary-light: #a29bfe;
  --primary-dark: #4834d4;
  --bg: #ffffff;
  --bg-secondary: #f8f9fa;
  --text: #1a1a2e;
  --text-secondary: #6c757d;
  --border: #e9ecef;
  --card-bg: #ffffff;
  --shadow: 0 2px 8px rgba(0,0,0,0.08);
  --shadow-hover: 0 8px 24px rgba(0,0,0,0.12);
  --radius: 12px;
  --nav-bg: rgba(255,255,255,0.9);
}

[data-theme="dark"] {
  --primary: #a29bfe;
  --primary-light: #6c5ce7;
  --primary-dark: #4834d4;
  --bg: #0f0f1a;
  --bg-secondary: #1a1a2e;
  --text: #e4e4e7;
  --text-secondary: #a1a1aa;
  --border: #27272a;
  --card-bg: #1a1a2e;
  --shadow: 0 2px 8px rgba(0,0,0,0.3);
  --shadow-hover: 0 8px 24px rgba(0,0,0,0.4);
  --nav-bg: rgba(15,15,26,0.9);
}

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

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Noto Sans KR', sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
  transition: background 0.3s, color 0.3s;
}

a { color: var(--primary); text-decoration: none; }
a:hover { color: var(--primary-light); }

.container { max-width: 1100px; margin: 0 auto; padding: 0 24px; }

/* Nav */
nav {
  position: sticky; top: 0; z-index: 100;
  background: var(--nav-bg);
  backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border);
}
nav .container {
  display: flex; align-items: center; justify-content: space-between;
  height: 64px;
}
nav .logo { font-weight: 700; font-size: 1.2rem; color: var(--text); }
nav .logo span { color: var(--primary); }
nav .nav-links { display: flex; gap: 24px; align-items: center; }
nav .nav-links a { color: var(--text-secondary); font-size: 0.9rem; font-weight: 500; }
nav .nav-links a:hover, nav .nav-links a.active { color: var(--primary); }
.theme-toggle {
  background: none; border: 1px solid var(--border); color: var(--text);
  width: 36px; height: 36px; border-radius: 8px; cursor: pointer;
  display: flex; align-items: center; justify-content: center;
  font-size: 1.1rem; transition: all 0.2s;
}
.theme-toggle:hover { border-color: var(--primary); color: var(--primary); }
.mobile-menu-btn {
  display: none; background: none; border: none; color: var(--text);
  font-size: 1.5rem; cursor: pointer;
}

/* Hero */
.hero {
  text-align: center; padding: 80px 0 60px;
  background: linear-gradient(180deg, var(--bg-secondary) 0%, var(--bg) 100%);
}
.hero h1 { font-size: 2.8rem; font-weight: 800; margin-bottom: 16px; }
.hero h1 span { background: linear-gradient(135deg, var(--primary), var(--primary-light)); -webkit-background-clip: text; -webkit-text-fill-color: transparent; }
.hero p { font-size: 1.15rem; color: var(--text-secondary); max-width: 640px; margin: 0 auto 32px; }
.hero-stats { display: flex; justify-content: center; gap: 48px; }
.hero-stats .stat { text-align: center; }
.hero-stats .stat .num { font-size: 1.8rem; font-weight: 700; color: var(--primary); }
.hero-stats .stat .label { font-size: 0.85rem; color: var(--text-secondary); }

/* Sections */
.section { padding: 60px 0; }
.section-title { font-size: 1.6rem; font-weight: 700; margin-bottom: 8px; }
.section-subtitle { color: var(--text-secondary); margin-bottom: 32px; }

/* Cards */
.grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(300px, 1fr)); gap: 20px; }
.card {
  background: var(--card-bg); border: 1px solid var(--border);
  border-radius: var(--radius); padding: 24px;
  transition: all 0.2s; box-shadow: var(--shadow);
}
.card:hover { box-shadow: var(--shadow-hover); transform: translateY(-2px); }
.card .icon { font-size: 1.5rem; margin-bottom: 12px; }
.card h3 { font-size: 1.05rem; margin-bottom: 8px; }
.card p { font-size: 0.9rem; color: var(--text-secondary); line-height: 1.5; }
.card .meta { font-size: 0.8rem; color: var(--text-secondary); margin-top: 12px; display: flex; gap: 12px; }

/* Course lists */
.course-category { margin-bottom: 40px; }
.course-category h3 { font-size: 1.2rem; margin-bottom: 16px; padding-bottom: 8px; border-bottom: 2px solid var(--primary); }
.course-list { list-style: none; }
.course-list li { border-bottom: 1px solid var(--border); }
.course-list li:last-child { border-bottom: none; }
.course-list .course-name { font-size: 0.95rem; }
.course-list .course-source { font-size: 0.8rem; color: var(--text-secondary); }
.course-link { display: flex; justify-content: space-between; align-items: center; width: 100%; padding: 10px 0; color: var(--text); text-decoration: none; }
.course-link:hover { color: var(--primary); }
.course-link:hover .course-name { color: var(--primary); }

/* Research table */
.research-table { width: 100%; border-collapse: collapse; }
.research-table th, .research-table td {
  text-align: left; padding: 12px 16px; border-bottom: 1px solid var(--border);
  font-size: 0.9rem;
}
.research-table th { font-weight: 600; color: var(--text-secondary); font-size: 0.8rem; text-transform: uppercase; letter-spacing: 0.05em; }
.research-table tr:hover td { background: var(--bg-secondary); }

/* Footer */
footer {
  border-top: 1px solid var(--border); padding: 32px 0;
  text-align: center; color: var(--text-secondary); font-size: 0.85rem;
}
footer a { color: var(--primary); }

/* About page */
.about-section { max-width: 720px; margin: 0 auto; }
.about-section h2 { font-size: 1.3rem; margin: 32px 0 16px; }
.about-section p { margin-bottom: 16px; color: var(--text-secondary); }
.about-section ul { margin: 0 0 16px 24px; color: var(--text-secondary); }
.about-section ul li { margin-bottom: 6px; }

/* Responsive */
@media (max-width: 768px) {
  nav .nav-links { display: none; }
  .mobile-menu-btn { display: block; }
  nav .nav-links.open {
    display: flex; flex-direction: column; position: absolute; top: 64px; left: 0; right: 0;
    background: var(--nav-bg); backdrop-filter: blur(12px);
    border-bottom: 1px solid var(--border); padding: 16px 24px; gap: 12px;
  }
  .hero h1 { font-size: 2rem; }
  .hero-stats { flex-wrap: wrap; gap: 24px; }
  .grid { grid-template-columns: 1fr; }
}