:root {
  --bg: #f4f6f9;
  --surface: #ffffff;
  --ink: #1e2732;
  --ink-soft: #5b6572;
  --line: #e2e7ee;
  --brand: #2d6cdf;
  --brand-dark: #1f57bd;
  --green: #2e8540;
  --green-soft: #e7f4ea;
  --red: #c0392b;
  --red-soft: #fbeaea;
  --amber: #b9770e;
  --shadow: 0 6px 22px rgba(23, 43, 77, 0.08);
  --radius: 14px;
  font-size: 16px;
}

* { box-sizing: border-box; }

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

/* ---------- Header / Footer ---------- */
.site-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 14px 24px;
  background: var(--surface);
  border-bottom: 1px solid var(--line);
  position: sticky;
  top: 0;
  z-index: 20;
}
.brand { display: flex; align-items: center; gap: 12px; }
.brand-mark {
  background: var(--brand);
  color: #fff;
  font-weight: 800;
  padding: 6px 10px;
  border-radius: 8px;
  letter-spacing: .5px;
}
.brand-title { font-weight: 600; }
.site-footer {
  text-align: center;
  color: var(--ink-soft);
  padding: 26px 16px 40px;
  font-size: .85rem;
}

/* ---------- Layout ---------- */
#app { max-width: 900px; margin: 0 auto; padding: 28px 20px 60px; }
.screen { animation: fade .25s ease; }
@keyframes fade { from { opacity: 0; transform: translateY(6px); } to { opacity: 1; transform: none; } }

/* ---------- Hero ---------- */
.hero { text-align: center; margin: 10px 0 22px; }
.hero h1 { font-size: 2rem; margin: 0 0 8px; }
.subtitle { color: var(--ink-soft); max-width: 620px; margin: 0 auto; }

.home-controls {
  display: flex;
  gap: 18px;
  justify-content: center;
  margin: 18px 0 26px;
  flex-wrap: wrap;
}
.switch { display: inline-flex; align-items: center; gap: 8px; color: var(--ink-soft); cursor: pointer; user-select: none; }
.switch input { width: 18px; height: 18px; accent-color: var(--brand); }

/* ---------- Chapter grid ---------- */
.chapter-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(220px, 1fr));
  gap: 16px;
}
.chapter-card {
  background: var(--surface);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 20px;
  cursor: pointer;
  box-shadow: var(--shadow);
  transition: transform .15s ease, box-shadow .15s ease, border-color .15s ease;
  text-align: left;
  display: flex;
  flex-direction: column;
  gap: 10px;
}
.chapter-card:hover { transform: translateY(-3px); box-shadow: 0 12px 28px rgba(23,43,77,.14); border-color: #c9d6ea; }
.chapter-card .cc-index { font-size: .8rem; font-weight: 700; color: var(--brand); letter-spacing: .4px; text-transform: uppercase; }
.chapter-card .cc-name { font-size: 1.15rem; font-weight: 700; }
.chapter-card .cc-meta { color: var(--ink-soft); font-size: .9rem; margin-top: auto; }
.chapter-card .cc-cta { margin-top: 6px; color: var(--brand); font-weight: 600; font-size: .9rem; }

/* ---------- Quiz ---------- */
.quiz-top { margin-bottom: 14px; }
.quiz-heading { display: flex; align-items: baseline; justify-content: space-between; gap: 12px; flex-wrap: wrap; }
.quiz-heading h2 { margin: 0; font-size: 1.4rem; }
.progress-label { color: var(--ink-soft); font-size: .9rem; white-space: nowrap; }
.progress-track { height: 8px; background: #e6ebf2; border-radius: 999px; overflow: hidden; margin-top: 10px; }
.progress-fill { height: 100%; width: 0%; background: linear-gradient(90deg, var(--brand), #4f8bef); transition: width .3s ease; }

.question-navigator {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin: 16px 0 18px;
}
.nav-chip {
  width: 34px; height: 34px;
  border-radius: 8px;
  border: 1px solid var(--line);
  background: var(--surface);
  color: var(--ink-soft);
  font-weight: 600;
  cursor: pointer;
  font-size: .85rem;
  transition: all .12s ease;
}
.nav-chip:hover { border-color: var(--brand); color: var(--brand); }
.nav-chip.answered { background: #eaf1fd; border-color: #b9cdf3; color: var(--brand-dark); }
.nav-chip.current { background: var(--brand); border-color: var(--brand); color: #fff; }

/* ---------- Question card ---------- */
.question-card {
  background: var(--surface);
  border: 1px solid var(--line);
  border-left: 6px solid var(--brand);
  border-radius: var(--radius);
  padding: 24px 26px;
  box-shadow: var(--shadow);
}
.q-number { font-size: .82rem; font-weight: 700; text-transform: uppercase; letter-spacing: .5px; color: var(--brand); }
.q-badge {
  display: inline-block;
  font-size: .72rem;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: .4px;
  padding: 2px 8px;
  border-radius: 999px;
  margin-left: 8px;
  background: #eef2f7;
  color: var(--ink-soft);
  vertical-align: middle;
}
.q-text { font-size: 1.25rem; font-weight: 600; margin: 10px 0 20px; }

.options { display: flex; flex-direction: column; gap: 10px; }
.option {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 13px 16px;
  border: 1.5px solid var(--line);
  border-radius: 10px;
  cursor: pointer;
  transition: border-color .12s ease, background .12s ease;
}
.option:hover { border-color: #b9cdf3; background: #f7faff; }
.option input { width: 18px; height: 18px; accent-color: var(--brand); flex: none; }
.option.selected { border-color: var(--brand); background: #eff5ff; }
.option .opt-text { flex: 1; }

/* graded states (instant feedback + review) */
.option.correct { border-color: var(--green); background: var(--green-soft); }
.option.incorrect { border-color: var(--red); background: var(--red-soft); }
.option .mark { font-weight: 800; flex: none; }
.option.correct .mark { color: var(--green); }
.option.incorrect .mark { color: var(--red); }

/* fill in the blank */
.blank-input {
  width: 100%;
  max-width: 420px;
  padding: 13px 16px;
  font-size: 1.05rem;
  border: 1.5px solid var(--line);
  border-radius: 10px;
  font-family: inherit;
  color: var(--ink);
}
.blank-input:focus { outline: none; border-color: var(--brand); box-shadow: 0 0 0 3px rgba(45,108,223,.15); }
.blank-hint { margin-top: 10px; font-size: .85rem; color: var(--ink-soft); }

.feedback {
  margin-top: 16px;
  padding: 12px 16px;
  border-radius: 10px;
  font-weight: 600;
  display: none;
}
.feedback.show { display: block; }
.feedback.good { background: var(--green-soft); color: var(--green); }
.feedback.bad { background: var(--red-soft); color: var(--red); }

/* ---------- Nav buttons ---------- */
.quiz-nav { display: flex; align-items: center; gap: 12px; margin-top: 22px; }
.quiz-nav .spacer { flex: 1; }

.btn {
  font: inherit;
  font-weight: 600;
  border: none;
  border-radius: 10px;
  padding: 11px 20px;
  cursor: pointer;
  transition: background .15s ease, transform .1s ease, box-shadow .15s ease;
}
.btn:active { transform: translateY(1px); }
.btn-primary { background: var(--brand); color: #fff; }
.btn-primary:hover { background: var(--brand-dark); }
.btn-success { background: var(--green); color: #fff; }
.btn-success:hover { background: #256b34; }
.btn-ghost { background: transparent; color: var(--ink-soft); border: 1px solid var(--line); }
.btn-ghost:hover { background: #eef2f7; color: var(--ink); }
.btn:disabled { opacity: .45; cursor: not-allowed; }

/* ---------- Results ---------- */
.results-summary {
  display: flex;
  gap: 26px;
  align-items: center;
  background: var(--surface);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  padding: 26px;
  box-shadow: var(--shadow);
  flex-wrap: wrap;
}
.score-ring {
  --pct: 0;
  width: 130px; height: 130px;
  border-radius: 50%;
  display: grid;
  place-items: center;
  flex: none;
  background: conic-gradient(var(--green) calc(var(--pct) * 1%), #e6ebf2 0);
}
.score-ring span {
  width: 100px; height: 100px;
  background: var(--surface);
  border-radius: 50%;
  display: grid;
  place-items: center;
  font-size: 1.8rem;
  font-weight: 800;
}
.results-meta h2 { margin: 0 0 4px; }
.score-line { color: var(--ink-soft); margin: 0 0 16px; font-size: 1.05rem; }
.results-buttons { display: flex; gap: 10px; flex-wrap: wrap; }

.review-list { margin-top: 22px; display: flex; flex-direction: column; gap: 16px; }
.review-card {
  background: var(--surface);
  border: 1px solid var(--line);
  border-left: 6px solid var(--line);
  border-radius: var(--radius);
  padding: 20px 24px;
  box-shadow: var(--shadow);
}
.review-card.correct { border-left-color: var(--green); }
.review-card.incorrect { border-left-color: var(--red); }
.review-status {
  display: inline-block;
  font-size: .72rem;
  font-weight: 800;
  text-transform: uppercase;
  letter-spacing: .5px;
  padding: 3px 10px;
  border-radius: 999px;
  color: #fff;
  margin-bottom: 8px;
}
.review-card.correct .review-status { background: var(--green); }
.review-card.incorrect .review-status { background: var(--red); }
.review-answer-line { margin-top: 8px; font-size: .95rem; }
.review-answer-line .lbl { color: var(--ink-soft); }
.review-answer-line .val-correct { color: var(--green); font-weight: 700; }
.review-answer-line .val-wrong { color: var(--red); font-weight: 700; text-decoration: line-through; }

@media (max-width: 560px) {
  .brand-title { display: none; }
  .q-text { font-size: 1.1rem; }
  .results-summary { justify-content: center; text-align: center; }
}