:root {
  --bg: #f5f8fa;
  --surface: #ffffff;
  --text: #1c2b33;
  --text-muted: #5b6b73;
  --primary: #0d7377;
  --primary-dark: #095357;
  --accent: #2ea0a8;
  --border: #dde6e9;
  --success: #1f8a4c;
  --success-bg: #e6f6ec;
  --error: #c23b3b;
  --error-bg: #fbeaea;
  --shadow: 0 1px 3px rgba(20, 40, 45, 0.08), 0 1px 2px rgba(20, 40, 45, 0.06);
  --radius: 12px;
  font-size: 16px;
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--text);
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  line-height: 1.5;
}

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

.site-header {
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  position: sticky;
  top: 0;
  z-index: 10;
}

.site-header .inner {
  max-width: 1000px;
  margin: 0 auto;
  padding: 14px 20px;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 16px;
  flex-wrap: wrap;
}

.brand {
  font-weight: 700;
  font-size: 1.15rem;
  color: var(--primary-dark);
  display: flex;
  align-items: center;
  gap: 8px;
}

.brand .dot { color: var(--accent); }

nav.main-nav {
  display: flex;
  gap: 18px;
  flex-wrap: wrap;
}

nav.main-nav a {
  color: var(--text-muted);
  font-weight: 600;
  font-size: 0.95rem;
  padding: 6px 2px;
  border-bottom: 2px solid transparent;
}

nav.main-nav a:hover,
nav.main-nav a.active {
  color: var(--primary-dark);
  text-decoration: none;
  border-bottom-color: var(--accent);
}

main {
  max-width: 1000px;
  margin: 0 auto;
  padding: 32px 20px 64px;
}

.hero {
  text-align: center;
  padding: 24px 0 8px;
}

.hero h1 {
  font-size: 2rem;
  margin: 0 0 8px;
  color: var(--primary-dark);
}

.hero p {
  color: var(--text-muted);
  max-width: 620px;
  margin: 0 auto;
  font-size: 1.05rem;
}

.subject-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(230px, 1fr));
  gap: 16px;
  margin: 28px 0;
}

.card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 20px;
}

.subject-card {
  display: flex;
  flex-direction: column;
  gap: 8px;
}

.subject-card .icon { font-size: 1.8rem; }

.subject-card h3 { margin: 0; font-size: 1.1rem; }

.subject-card p {
  color: var(--text-muted);
  font-size: 0.9rem;
  margin: 0 0 8px;
  flex-grow: 1;
}

.subject-card .links {
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
  font-size: 0.85rem;
}

.subject-card .links a {
  background: var(--bg);
  border: 1px solid var(--border);
  border-radius: 8px;
  padding: 5px 10px;
  color: var(--primary-dark);
  font-weight: 600;
}

.subject-card .links a:hover {
  background: var(--accent);
  color: white;
  text-decoration: none;
  border-color: var(--accent);
}

.action-row {
  display: flex;
  gap: 14px;
  flex-wrap: wrap;
  justify-content: center;
  margin: 20px 0 8px;
}

.btn {
  display: inline-block;
  background: var(--primary);
  color: white;
  border: none;
  border-radius: 8px;
  padding: 11px 20px;
  font-size: 0.95rem;
  font-weight: 600;
  cursor: pointer;
}

.btn:hover { background: var(--primary-dark); text-decoration: none; }

.btn.secondary {
  background: var(--surface);
  color: var(--primary-dark);
  border: 1px solid var(--border);
}

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

.btn:disabled { opacity: 0.5; cursor: not-allowed; }

.subject-tabs {
  display: flex;
  gap: 8px;
  flex-wrap: wrap;
  margin-bottom: 24px;
}

.subject-tabs a {
  padding: 7px 14px;
  border-radius: 20px;
  border: 1px solid var(--border);
  background: var(--surface);
  color: var(--text-muted);
  font-size: 0.9rem;
  font-weight: 600;
}

.subject-tabs a.active {
  background: var(--primary);
  color: white;
  border-color: var(--primary);
  text-decoration: none;
}

.review-section {
  margin-bottom: 22px;
}

.review-section h3 {
  color: var(--primary-dark);
  margin-bottom: 8px;
}

.review-section ul {
  margin: 0;
  padding-left: 22px;
}

.review-section li { margin-bottom: 6px; }

.quiz-card {
  margin-bottom: 18px;
}

.quiz-card .q-text {
  font-weight: 600;
  margin-bottom: 12px;
}

.choice {
  display: block;
  width: 100%;
  text-align: left;
  padding: 10px 14px;
  margin-bottom: 8px;
  border: 1px solid var(--border);
  border-radius: 8px;
  background: var(--surface);
  cursor: pointer;
  font-size: 0.95rem;
  color: var(--text);
}

.choice:hover { border-color: var(--accent); }

.choice.correct { background: var(--success-bg); border-color: var(--success); color: var(--success); font-weight: 600; }
.choice.incorrect { background: var(--error-bg); border-color: var(--error); color: var(--error); font-weight: 600; }
.choice.disabled { cursor: default; }

.explain {
  font-size: 0.9rem;
  color: var(--text-muted);
  background: var(--bg);
  border-left: 3px solid var(--accent);
  padding: 8px 12px;
  border-radius: 6px;
  margin-top: 4px;
  display: none;
}

.explain.show { display: block; }

.progress-bar-track {
  background: var(--border);
  border-radius: 20px;
  height: 8px;
  overflow: hidden;
  margin: 6px 0 18px;
}

.progress-bar-fill {
  background: var(--accent);
  height: 100%;
  transition: width 0.2s ease;
}

.timer {
  font-variant-numeric: tabular-nums;
  font-weight: 700;
  font-size: 1.1rem;
  color: var(--primary-dark);
}

.top-bar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  flex-wrap: wrap;
  margin-bottom: 8px;
}

.score-summary {
  text-align: center;
  padding: 24px;
}

.score-summary .big-score {
  font-size: 3rem;
  font-weight: 800;
  color: var(--primary-dark);
}

.flashcard-wrap {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 18px;
}

.flashcard {
  width: 100%;
  max-width: 480px;
  min-height: 270px;
  perspective: 1200px;
  cursor: pointer;
}

.flashcard-inner {
  position: relative;
  width: 100%;
  height: 270px;
  transition: transform 0.5s;
  transform-style: preserve-3d;
}

.flashcard.flipped .flashcard-inner { transform: rotateY(180deg); }

.flashcard-face {
  position: absolute;
  inset: 0;
  backface-visibility: hidden;
  border-radius: var(--radius);
  border: 1px solid var(--border);
  box-shadow: var(--shadow);
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  text-align: center;
  padding: 24px;
  font-size: 1.2rem;
  font-weight: 600;
  gap: 14px;
}

.flashcard-face.front { background: var(--surface); color: var(--text); }
.flashcard-face.back {
  background: var(--primary);
  color: white;
  transform: rotateY(180deg);
  font-weight: 500;
  font-size: 1.05rem;
}

.flash-icon { color: var(--accent); line-height: 0; }
.flash-icon-svg { width: 100%; max-width: 190px; height: auto; }
.flash-term { font-size: 1.2rem; font-weight: 600; }

.flashcard-controls {
  display: flex;
  align-items: center;
  gap: 16px;
}

.flashcard-counter { color: var(--text-muted); font-size: 0.9rem; }

table.history-table {
  width: 100%;
  border-collapse: collapse;
  background: var(--surface);
  border-radius: var(--radius);
  overflow: hidden;
  box-shadow: var(--shadow);
}

table.history-table th, table.history-table td {
  padding: 10px 14px;
  text-align: left;
  border-bottom: 1px solid var(--border);
  font-size: 0.92rem;
}

table.history-table th { background: var(--bg); color: var(--text-muted); }

.empty-state {
  text-align: center;
  color: var(--text-muted);
  padding: 40px 20px;
}

.site-footer {
  text-align: center;
  color: var(--text-muted);
  font-size: 0.85rem;
  padding: 24px 20px 40px;
}

@media (max-width: 600px) {
  .hero h1 { font-size: 1.5rem; }
  main { padding: 20px 14px 48px; }
}