/* 수학 단원 콘텐츠 공용 스타일 */
:root {
  --primary: #3b5bdb;
  --primary-light: #eef1fd;
  --text: #1f2430;
  --muted: #6b7280;
  --bg: #f7f8fb;
  --card-bg: #ffffff;
  --border: #e5e7eb;
}

* { box-sizing: border-box; }

body {
  margin: 0;
  font-family: "Apple SD Gothic Neo", "Malgun Gothic", "Noto Sans KR", sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.6;
}

header.page-header {
  background: linear-gradient(135deg, var(--primary), #6a8dff);
  color: #fff;
  padding: 48px 24px;
  text-align: center;
}

header.page-header h1 {
  margin: 0 0 8px;
  font-size: 2rem;
}

header.page-header p {
  margin: 0;
  opacity: 0.9;
}

/* ---------- 단원 전환 버튼 ---------- */
.unit-nav {
  margin: 20px auto 0;
  display: flex;
  flex-wrap: wrap;
  justify-content: center;
  gap: 10px;
}

.unit-btn {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 2px;
  min-width: 118px;
  padding: 9px 18px;
  border: 2px solid rgba(255, 255, 255, 0.55);
  border-radius: 999px;
  background: rgba(255, 255, 255, 0.12);
  color: #fff;
  font: inherit;
  line-height: 1.25;
  cursor: pointer;
  transition: background 0.15s, color 0.15s, border-color 0.15s;
}

.unit-btn strong { font-size: 1rem; }
.unit-btn span   { font-size: 0.78rem; opacity: 0.85; }

.unit-btn:hover { background: rgba(255, 255, 255, 0.26); }

.unit-btn.on {
  background: #fff;
  border-color: #fff;
  color: var(--primary);
  font-weight: 700;
}

.unit-btn.on span { opacity: 0.75; }

main {
  max-width: 960px;
  margin: 0 auto;
  padding: 32px 24px 64px;
}

.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(240px, 1fr));
  gap: 20px;
  margin-top: 24px;
}

.card {
  display: block;
  background: var(--card-bg);
  border: 1px solid var(--border);
  border-radius: 12px;
  padding: 20px;
  text-decoration: none;
  color: var(--text);
  transition: transform 0.15s ease, box-shadow 0.15s ease;
}

.card:hover {
  transform: translateY(-4px);
  box-shadow: 0 8px 20px rgba(59, 91, 219, 0.15);
  border-color: var(--primary);
}

.card .badge {
  display: inline-block;
  font-size: 0.75rem;
  color: var(--primary);
  background: var(--primary-light);
  padding: 2px 10px;
  border-radius: 999px;
  margin-bottom: 10px;
}

.card h2 {
  margin: 0 0 6px;
  font-size: 1.15rem;
}

.card p {
  margin: 0;
  color: var(--muted);
  font-size: 0.9rem;
}

.back-link {
  display: inline-block;
  margin-bottom: 24px;
  color: var(--primary);
  text-decoration: none;
  font-weight: 600;
}

.back-link:hover {
  text-decoration: underline;
}

.empty-notice {
  margin-top: 24px;
  padding: 20px;
  background: var(--card-bg);
  border: 1px dashed var(--border);
  border-radius: 12px;
  color: var(--muted);
  text-align: center;
}