:root {
  --bg:          #0f1117;
  --surface:     #1a1d2e;
  --surface2:    #252839;
  --accent:      #4f7fff;
  --accent-dim:  rgba(79,127,255,.12);
  --success:     #22c55e;
  --success-dim: rgba(34,197,94,.12);
  --danger:      #ef4444;
  --danger-dim:  rgba(239,68,68,.12);
  --text:        #e2e8f0;
  --muted:       #94a3b8;
  --border:      #2e3347;
  --radius:      10px;
}

body.light {
  --bg:          #f1f4fb;
  --surface:     #ffffff;
  --surface2:    #e8ecf5;
  --accent:      #3b6ef0;
  --accent-dim:  rgba(59,110,240,.1);
  --success:     #16a34a;
  --success-dim: rgba(22,163,74,.1);
  --danger:      #dc2626;
  --danger-dim:  rgba(220,38,38,.1);
  --text:        #0f172a;
  --muted:       #64748b;
  --border:      #cdd3e0;
}

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

/* Suppress tap flash and double-tap zoom delay on all interactive elements */
button, [role="button"], .topic-card, .answer-opt {
  -webkit-tap-highlight-color: transparent;
  touch-action: manipulation;
}

body {
  font-family: 'Segoe UI', system-ui, -apple-system, sans-serif;
  background: var(--bg);
  color: var(--text);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  align-items: center;
}

/* ── Header ── */
header {
  width: 100%;
  /* Safe area: top for notch, sides for landscape mode */
  padding: max(.85rem, env(safe-area-inset-top)) max(1.5rem, env(safe-area-inset-right)) .85rem max(1.5rem, env(safe-area-inset-left));
  background: var(--surface);
  border-bottom: 1px solid var(--border);
  display: flex;
  align-items: center;
  gap: .65rem;
  position: sticky;
  top: 0;
  z-index: 10;
}
header svg { flex-shrink: 0; }
header span {
  font-size: 1rem;
  font-weight: 600;
  /* Prevent overflow on narrow screens */
  overflow: hidden;
  text-overflow: ellipsis;
  white-space: nowrap;
  min-width: 0;
}

/* Hide title text on very small screens – icon + toggle button are enough */
@media (max-width: 480px) {
  header span { display: none; }
}

.btn-theme {
  margin-left: auto;
  background: none;
  border: 1px solid var(--border);
  border-radius: 8px;
  /* Minimum 44×44 px touch target */
  width: 44px;
  height: 44px;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  flex-shrink: 0;
  transition: border-color .15s, background .15s;
  color: var(--text);
}

/* ── Screens ── */
.screen {
  display: none;
  width: 100%;
  max-width: 780px;
  padding: 2rem 1rem 3rem;
}
.screen.active { display: block; }

.page-title    { font-size: 1.75rem; font-weight: 700; margin-bottom: .35rem; }
.page-subtitle { color: var(--muted); margin-bottom: 2rem; line-height: 1.5; }

/* ── Topic cards ── */
#topics-grid {
  display: grid;
  /* min() ensures cards never exceed available width on narrow viewports */
  grid-template-columns: repeat(auto-fill, minmax(min(310px, 100%), 1fr));
  gap: 1rem;
}
.topic-card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.4rem 1.5rem;
  cursor: pointer;
  transition: border-color .18s, transform .18s, box-shadow .18s;
  display: flex;
  flex-direction: column;
  gap: .45rem;
}
.topic-card:active { opacity: .82; transform: scale(.99); transition: transform .08s, opacity .08s; }
.tc-num   { font-size: .75rem; color: var(--accent); font-weight: 700; text-transform: uppercase; letter-spacing: .06em; }
.tc-name  { font-size: 1rem; font-weight: 600; line-height: 1.35; }
.tc-count { font-size: .82rem; color: var(--muted); }
.tc-arrow { margin-top: .5rem; font-size: .8rem; color: var(--accent); font-weight: 600; }

/* ── Quiz topbar + progress ── */
.quiz-topbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: 1.25rem;
}
.btn-back {
  background: none;
  border: 1px solid var(--border);
  color: var(--muted);
  /* Minimum 44 px touch target via min-height */
  padding: 0 .9rem;
  min-height: 44px;
  border-radius: 6px;
  cursor: pointer;
  font-size: .82rem;
  transition: border-color .15s, color .15s, background .15s;
}
.btn-back:active { background: var(--accent-dim); color: var(--accent); border-color: var(--accent); }

.progress-label { font-size: .85rem; color: var(--muted); }

.progress-track {
  width: 100%;
  height: 4px;
  background: var(--border);
  border-radius: 2px;
  margin-bottom: 2rem;
  overflow: hidden;
}
.progress-fill {
  height: 100%;
  background: var(--accent);
  border-radius: 2px;
  transition: width .35s ease;
}

/* ── Question card ── */
.question-card {
  background: var(--surface);
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 1.75rem;
  margin-bottom: 1.25rem;
}
.q-text {
  font-size: 1.1rem;
  font-weight: 600;
  line-height: 1.55;
  margin-bottom: 1.5rem;
}

/* ── Answer options ── */
.answers { display: flex; flex-direction: column; gap: .65rem; }

.answer-opt {
  display: flex;
  align-items: flex-start;
  gap: .75rem;
  padding: .85rem 1rem;
  background: var(--surface2);
  border: 1px solid var(--border);
  border-radius: 8px;
  cursor: pointer;
  transition: border-color .15s, background .15s, opacity .15s;
  user-select: none;
}
.answer-opt.sel { border-color: var(--accent); background: var(--accent-dim); }

/* Touch active state for answer options */
.answer-opt:not(.locked):active { opacity: .75; }

/* feedback states */
.answer-opt.locked       { cursor: default; }
.answer-opt.fb-correct   { border-color: var(--success) !important; background: var(--success-dim) !important; }
.answer-opt.fb-wrong-sel { border-color: var(--danger)  !important; background: var(--danger-dim)  !important; }
.answer-opt.fb-missed    { border-color: var(--success) !important; background: transparent !important; opacity: .75; }
.answer-opt.fb-neutral   { opacity: .4; }

/* checkbox icon */
.cb-box {
  width: 18px; height: 18px; min-width: 18px;
  border: 2px solid var(--border);
  border-radius: 4px;
  margin-top: 2px;
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 11px;
  font-weight: 700;
  transition: all .15s;
  flex-shrink: 0;
}
.answer-opt.sel          .cb-box { border-color: var(--accent);  background: var(--accent);  color: #fff; }
.answer-opt.fb-correct   .cb-box { border-color: var(--success); background: var(--success); color: #fff; }
.answer-opt.fb-wrong-sel .cb-box { border-color: var(--danger);  background: var(--danger);  color: #fff; }
.answer-opt.fb-missed    .cb-box { border-color: var(--success); background: transparent; color: var(--success); }

/* answer content */
.cb-content { flex: 1; }
.cb-label   { line-height: 1.45; font-size: .95rem; }
.cb-expl {
  display: none;
  margin-top: .4rem;
  font-size: .8rem;
  line-height: 1.5;
  color: var(--muted);
}
.answers.revealed .cb-expl { display: block; }

/* ── Feedback banner ── */
.feedback-banner {
  display: none;
  align-items: center;
  gap: .65rem;
  padding: .8rem 1rem;
  border-radius: 8px;
  margin-bottom: 1.25rem;
  font-weight: 600;
  font-size: .95rem;
}
.feedback-banner.ok  { display: flex; background: var(--success-dim); color: var(--success); border: 1px solid var(--success); }
.feedback-banner.err { display: flex; background: var(--danger-dim);  color: var(--danger);  border: 1px solid var(--danger);  }

/* ── Submit button ── */
.btn-submit {
  width: 100%;
  padding: .9rem;
  background: var(--accent);
  color: #fff;
  border: none;
  border-radius: 8px;
  font-size: 1rem;
  font-weight: 600;
  cursor: pointer;
  transition: background .15s, filter .15s;
}
.btn-submit:active { filter: brightness(.88); }

/* Sticky on mobile: always visible at bottom even when feedback content is long */
@media (max-width: 640px) {
  .btn-submit {
    position: sticky;
    bottom: 0;
    /* Grow bottom padding to clear the iOS home bar */
    padding-bottom: max(.9rem, env(safe-area-inset-bottom));
    border-radius: 0;
    box-shadow: 0 -6px 20px rgba(0, 0, 0, .2);
    z-index: 5;
  }
}

/* ── Results ── */
.results-top { text-align: center; margin-bottom: 2.25rem; }
.score-ring {
  width: 120px; height: 120px;
  border-radius: 50%;
  border: 4px solid var(--accent);
  margin: 0 auto 1rem;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
}
.score-num { font-size: 2rem; font-weight: 700; color: var(--accent); line-height: 1; }
.score-sub { font-size: .75rem; color: var(--muted); margin-top: .2rem; }
.res-title    { font-size: 1.35rem; font-weight: 700; margin-bottom: .25rem; }
.res-subtitle { color: var(--muted); font-size: .9rem; }

#results-list { display: flex; flex-direction: column; gap: .75rem; margin-bottom: 1.5rem; }

.res-item {
  background: var(--surface);
  border: 1px solid var(--border);
  border-left-width: 3px;
  border-radius: var(--radius);
  overflow: hidden;
}
.res-item.ok  { border-left-color: var(--success); }
.res-item.err { border-left-color: var(--danger);  }

.res-row {
  display: flex;
  align-items: flex-start;
  gap: .7rem;
  padding: .9rem 1.1rem;
}
.res-icon { font-size: 1rem; min-width: 18px; margin-top: 1px; font-style: normal; }
.res-q    { flex: 1; font-size: .9rem; line-height: 1.45; }

/* Minimum 44×44 px touch target */
.btn-explain {
  background: none;
  border: 1px solid var(--border);
  color: var(--muted);
  min-width: 44px;
  min-height: 44px;
  border-radius: 50%;
  cursor: pointer;
  font-size: .85rem;
  font-weight: 700;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: border-color .15s, color .15s, background .15s;
  flex-shrink: 0;
}
.btn-explain:active { border-color: var(--accent); color: var(--accent); background: var(--accent-dim); }
.btn-explain.open   { background: var(--accent); border-color: var(--accent); color: #fff; }

.exp-panel {
  display: none;
  padding: 0 1.1rem 1rem;
  border-top: 1px solid var(--border);
}
.exp-panel.open { display: block; }

.exp-header {
  font-size: .75rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: .06em;
  color: var(--muted);
  padding: .75rem 0 .5rem;
}
.exp-answer {
  display: flex;
  align-items: flex-start;
  gap: .6rem;
  padding: .55rem 0;
  font-size: .85rem;
}
.exp-answer + .exp-answer { border-top: 1px solid var(--border); }
.exp-marker     { min-width: 16px; font-size: .85rem; font-weight: 700; margin-top: 1px; }
.exp-marker.ok  { color: var(--success); }
.exp-marker.err { color: var(--danger); }
.exp-body-text  { font-weight: 500; line-height: 1.35; margin-bottom: .2rem; }
.tag-selected {
  display: inline-block;
  margin-left: .4rem;
  font-size: .75rem;
  font-weight: 600;
  background: var(--accent-dim);
  color: var(--accent);
  padding: .05rem .35rem;
  border-radius: 4px;
  vertical-align: middle;
}
.exp-body-expl { color: var(--muted); font-size: .8rem; line-height: 1.45; }

/* ── Restart button ── */
.btn-restart {
  width: 100%;
  padding: .85rem;
  background: var(--surface);
  color: var(--text);
  border: 1px solid var(--border);
  border-radius: 8px;
  font-size: .95rem;
  font-weight: 600;
  cursor: pointer;
  transition: border-color .15s, color .15s, background .15s;
}
.btn-restart:active { background: var(--accent-dim); border-color: var(--accent); color: var(--accent); }

/* ── Hover states – only on devices with a real pointer (not touch) ── */
@media (hover: hover) {
  .topic-card:hover {
    border-color: var(--accent);
    transform: translateY(-2px);
    box-shadow: 0 6px 24px rgba(79,127,255,.18);
  }
  .btn-back:hover       { border-color: var(--muted); color: var(--text); }
  .btn-theme:hover      { border-color: var(--accent); background: var(--accent-dim); }
  .answer-opt:not(.locked):hover { border-color: var(--accent); background: var(--accent-dim); }
  .btn-submit:hover     { background: #6b92ff; }
  .btn-explain:hover    { border-color: var(--accent); color: var(--accent); }
  .btn-restart:hover    { border-color: var(--accent); color: var(--accent); }
}

/* ── Responsive ── */
@media (max-width: 580px) {
  .screen        { padding: 1.5rem .75rem 2.5rem; }
  .question-card { padding: 1.25rem; }
  .page-title    { font-size: 1.4rem; }
  .score-ring    { width: 100px; height: 100px; }
  .score-num     { font-size: 1.7rem; }
}