:root {
  --bg: #f6f7f9;
  --panel: #ffffff;
  --panel-2: #f0f2f5;
  --text: #1f2937;
  --muted: #667085;
  --border: #d7dce3;
  --accent: #2989b3;
  --accent-dark: #206f91;
  --accent-soft: rgba(41, 137, 179, 0.12);
  --pi: #7a5aa6;
  --pi-soft: rgba(122, 90, 166, 0.16);
  --success: #287a4b;
  --warning: #9a6418;
  --danger: #a33b3b;
  --shadow: 0 12px 32px rgba(15, 23, 42, 0.08);
}

body.dark {
  --bg: #13171b;
  --panel: #1d2329;
  --panel-2: #252c33;
  --text: #eef2f6;
  --muted: #aab4bf;
  --border: #39434d;
  --accent: #56afd2;
  --accent-dark: #82c8e2;
  --accent-soft: rgba(86, 175, 210, 0.16);
  --pi: #b49bd6;
  --pi-soft: rgba(180, 155, 214, 0.18);
  --shadow: none;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
  background: var(--bg);
  color: var(--text);
  line-height: 1.55;
}

button, select, input {
  font: inherit;
}

button, select {
  border-radius:var(--r-md,14px);
}

button {
  cursor: pointer;
}

.container {
  width: min(1120px, calc(100% - 32px));
  margin: 0 auto;
}

.site-header {
  padding: 44px 0 30px;
  background:
    radial-gradient(circle at top left, var(--accent-soft), transparent 42%),
    var(--panel);
  border-bottom: 1px solid var(--border);
}

.header-inner {
  display: flex;
  justify-content: space-between;
  gap: 24px;
  align-items: flex-start;
}

.eyebrow {
  margin: 0 0 8px;
  color: var(--accent-dark);
  font-weight: 700;
  letter-spacing: .04em;
  text-transform: uppercase;
  font-size: .82rem;
}

h1, h2, h3, p { margin-top: 0; }

h1 {
  margin-bottom: 10px;
  font-size: clamp(2rem, 4vw, 3.3rem);
  line-height: 1.05;
}

h2 { font-size: 1.35rem; }

h3 { font-size: 1.05rem; }

.subtitle {
  margin-bottom: 0;
  max-width: 760px;
  color: var(--muted);
  font-size: 1.05rem;
}

.tabs {
  display: flex;
  gap: 8px;
  flex-wrap: wrap;
  padding: 24px 0 10px;
}

.tab {
  padding: 10px 16px;
  border: 1px solid var(--border);
  background: var(--panel);
  color: var(--text);
}

.tab.active {
  background: var(--accent);
  color: white;
  border-color: var(--accent);
}

.tab-panel {
  display: none;
  padding: 18px 0 50px;
}

.tab-panel.active { display: block; }

.control-panel {
  display: grid;
  grid-template-columns: minmax(240px, 1.4fr) repeat(3, 1fr);
  gap: 14px;
  padding: 18px;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius:var(--r-lg,18px);
  box-shadow: var(--shadow);
}

.control-panel label > span:first-child {
  display: block;
  margin-bottom: 6px;
  font-size: .9rem;
  color: var(--muted);
}

select {
  width: 100%;
  padding: 10px 12px;
  border: 1px solid var(--border);
  background: var(--bg);
  color: var(--text);
}

.switch-row {
  display: flex;
  align-items: center;
  gap: 10px;
  padding-top: 24px;
}

.switch-row input {
  width: 18px;
  height: 18px;
  accent-color: var(--accent);
}

.stats-grid {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 14px;
  margin: 18px 0;
}

.stat-card {
  padding: 16px;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius:var(--r-lg,18px);
}

.stat-card strong {
  display: block;
  margin: 4px 0;
  font-size: 1.9rem;
}

.stat-card small,
.stat-label,
.muted {
  color: var(--muted);
}

.visual-section,
.info-card,
.key-rule,
.concept-note,
.quiz-container {
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius:var(--r-lg,18px);
}

.visual-section {
  padding: 18px;
}

.visual-heading {
  display: flex;
  justify-content: space-between;
  gap: 18px;
  align-items: center;
}

.badge {
  display: inline-block;
  padding: 6px 10px;
  margin: 0 6px 6px 0;
  border-radius: 999px;
  color: white;
  background: var(--accent);
  font-size: .86rem;
  font-weight: 700;
}

.badge.outline {
  color: var(--accent-dark);
  background: var(--accent-soft);
}

.diagram-wrap {
  margin: 10px 0 4px;
  overflow: hidden;
  border-radius:var(--r-md,14px);
  background:
    linear-gradient(var(--panel-2) 1px, transparent 1px),
    linear-gradient(90deg, var(--panel-2) 1px, transparent 1px);
  background-size: 28px 28px;
}

#bondDiagram {
  width: 100%;
  display: block;
  min-height: 360px;
}

#bondCanvas {
  width: 100%;
  height: auto;
  display: block;
  touch-action: pan-y;
  border-radius:var(--r-md,14px);
  cursor: grab;
}
#bondCanvas:active { cursor: grabbing; }
#bondCanvas:focus-visible { outline: 2px solid var(--accent); outline-offset: 2px; }

.atom {
  fill: var(--panel);
  stroke: var(--text);
  stroke-width: 3;
}

.atom-label {
  fill: var(--text);
  font-size: 28px;
  font-weight: 800;
}

.bond-lines line {
  stroke: var(--text);
  stroke-width: 6;
  stroke-linecap: round;
}

.substituent-bonds line {
  stroke: var(--muted);
  stroke-width: 3.4;
  stroke-linecap: round;
}

.hydrogen-layer text {
  fill: var(--text);
  font-size: 21px;
  font-weight:700;
}

.orbital {
  transform-box: fill-box;
  transform-origin: center;
}

.sigma-orbital {
  fill: var(--accent-soft);
  stroke: var(--accent);
  stroke-width: 2.5;
}

.pi-orbital,
.pi2-orbital {
  fill: var(--pi-soft);
  stroke: var(--pi);
  stroke-width: 2.5;
}

.pi2-orbital { fill: none; stroke-width: 9; opacity: .45; }

.angle-arc {
  fill: none;
  stroke: var(--accent);
  stroke-width: 3;
}

.angle-text,
.bond-labels text {
  fill: var(--accent-dark);
  font-weight: 800;
  font-size: 19px;
}

.is-animated .sigma-orbital {
  animation: sigmaPulse 1.9s ease-in-out infinite alternate;
}

.is-animated .pi-orbital,
.is-animated .pi2-orbital {
  animation: piPulse 1.5s ease-in-out infinite alternate;
}

.rotate-demo #substituentBonds,
.rotate-demo #hydrogenLayer {
  transform-box: fill-box;
  transform-origin: center;
  animation: rotateDemo 1.2s ease-in-out;
}

.blocked-demo #substituentBonds {
  transform-box: fill-box;
  transform-origin: center;
  animation: blockedDemo .45s ease-in-out 2;
}

@keyframes sigmaPulse {
  from { opacity: .62; transform: scaleX(.96); }
  to { opacity: 1; transform: scaleX(1.035); }
}

@keyframes piPulse {
  from { opacity: .55; transform: scale(.96); }
  to { opacity: 1; transform: scale(1.03); }
}

@keyframes rotateDemo {
  0% { transform: rotate(0); }
  50% { transform: rotate(22deg); }
  100% { transform: rotate(0); }
}

@keyframes blockedDemo {
  0%, 100% { transform: rotate(0); }
  50% { transform: rotate(4deg); }
}

.hidden { display: none !important; }

.action-row {
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
  margin-top: 8px;
}

.primary-btn,
.secondary-btn {
  padding: 10px 14px;
  border: 1px solid var(--accent);
  font-weight: 700;
}

.primary-btn {
  background: var(--accent);
  color: white;
}

.primary-btn:hover { background: var(--accent-dark); }

.secondary-btn {
  background: var(--panel);
  color: var(--accent-dark);
}

.feedback {
  margin: 12px 0 0;
  color: var(--muted);
}

.explanation-grid {
  display: grid;
  grid-template-columns: 1.1fr .9fr;
  gap: 18px;
  margin-top: 18px;
}

.info-card {
  padding: 18px;
}

.property-list {
  margin: 0;
}

.property-list div {
  display: flex;
  justify-content: space-between;
  gap: 18px;
  padding: 10px 0;
  border-bottom: 1px solid var(--border);
}

.property-list div:last-child { border-bottom: 0; }

.property-list dt { color: var(--muted); }
.property-list dd { margin: 0; font-weight: 700; text-align: right; }

.section-heading {
  margin-bottom: 18px;
}

.section-heading p {
  margin-bottom: 0;
  color: var(--muted);
}

.table-wrap {
  overflow-x: auto;
  border: 1px solid var(--border);
  border-radius:var(--r-lg,18px);
}

table {
  width: 100%;
  border-collapse: collapse;
  min-width: 900px;
  background: var(--panel);
}

th, td {
  padding: 13px 14px;
  border-bottom: 1px solid var(--border);
  text-align: left;
}

th {
  background: var(--panel-2);
  font-size: .9rem;
}

tbody tr:last-child td { border-bottom: 0; }

.key-rule,
.concept-note {
  margin-top: 18px;
  padding: 16px 18px;
  border-left: 5px solid var(--accent);
}

.learning-flow {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 14px;
}

.learning-flow article {
  padding: 18px;
  background: var(--panel);
  border: 1px solid var(--border);
  border-radius:var(--r-lg,18px);
}

.step-number {
  display: inline-grid;
  place-items: center;
  width: 30px;
  height: 30px;
  margin-bottom: 10px;
  border-radius: 50%;
  background: var(--accent);
  color: white;
  font-weight: 800;
}

.quiz-container {
  padding: 18px;
}

.quiz-question {
  padding: 18px 0;
  border-bottom: 1px solid var(--border);
}

.quiz-question:first-child { padding-top: 0; }
.quiz-question:last-child { border-bottom: 0; padding-bottom: 0; }

.quiz-question fieldset {
  border: 0;
  padding: 0;
  margin: 0;
}

.quiz-question legend {
  font-weight: 700;
  margin-bottom: 10px;
}

.quiz-option {
  display: flex;
  align-items: flex-start;
  gap: 9px;
  margin: 8px 0;
}

.quiz-option input {
  margin-top: 4px;
  accent-color: var(--accent);
}

.quiz-actions {
  display: flex;
  gap: 10px;
  margin: 16px 0;
  flex-wrap: wrap;
}

.quiz-result {
  min-height: 24px;
  font-weight: 700;
}

.quiz-result.good { color: var(--success); }
.quiz-result.mid { color: var(--warning); }
.quiz-result.low { color: var(--danger); }

.qfb.good { color: var(--success); font-weight:700; }
.qfb.bad { color: var(--danger); font-weight:700; }
.feedback.bad { color: var(--danger); font-weight:700; }

footer {
  padding: 26px 0;
  color: var(--muted);
  border-top: 1px solid var(--border);
  background: var(--panel);
}

footer p { margin: 0; }

@media (max-width: 860px) {
  .control-panel,
  .stats-grid,
  .explanation-grid,
  .learning-flow {
    grid-template-columns: 1fr 1fr;
  }

  .header-inner,
  .visual-heading {
    align-items: flex-start;
    flex-direction: column;
  }

  .switch-row { padding-top: 0; }
}

@media (max-width: 580px) {
  .container { width: min(100% - 20px, 1120px); }

  .control-panel,
  .stats-grid,
  .explanation-grid,
  .learning-flow {
    grid-template-columns: 1fr;
  }

  .site-header { padding-top: 30px; }

  #bondDiagram { min-height: 280px; }

  .tabs {
    display: grid;
    grid-template-columns: 1fr 1fr;
  }

  .tab { width: 100%; }

  .property-list div {
    flex-direction: column;
    gap: 4px;
  }

  .property-list dd { text-align: left; }
}