:root {
  color-scheme: light;
  --bg: #f2efe9;
  --bg-deep: #e6ded1;
  --ink: #1c1b1a;
  --muted: #5e5a55;
  --accent: #d66b2d;
  --accent-dark: #a54c1c;
  --cool: #0f3d3e;
  --panel: #fbfaf7;
  --panel-shadow: 0 30px 60px -40px rgba(0, 0, 0, 0.35);
  --stroke: rgba(28, 27, 26, 0.12);
}

/* course breadcrumb navigation */
.course-nav {
  background: var(--bg-deep);
  border-bottom: 1px solid var(--stroke);
  padding: 0.75rem 8vw;
}

.course-nav .nav-breadcrumb {
  display: flex;
  align-items: center;
  gap: 0.5rem;
  font-size: 0.85rem;
}

.course-nav a {
  color: var(--muted);
  text-decoration: none;
  transition: color 0.2s;
}

.course-nav a:hover {
  color: var(--accent);
}

.course-nav .sep {
  color: var(--muted);
  opacity: 0.5;
}

.course-nav .nav-badge {
  font-family: "JetBrains Mono", monospace;
  font-size: 0.75rem;
  font-weight: 600;
  color: var(--accent);
  background: rgba(214, 107, 45, 0.1);
  padding: 0.2rem 0.5rem;
  border-radius: 4px;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: "Space Grotesk", sans-serif;
  color: var(--ink);
  background: radial-gradient(circle at top, #ffffff, var(--bg));
  min-height: 100vh;
}

.noise {
  position: fixed;
  inset: 0;
  background-image: url("data:image/svg+xml,%3Csvg width='120' height='120' xmlns='http://www.w3.org/2000/svg'%3E%3Cfilter id='n'%3E%3CfeTurbulence type='fractalNoise' baseFrequency='0.8' numOctaves='2'/%3E%3C/filter%3E%3Crect width='120' height='120' filter='url(%23n)' opacity='0.06'/%3E%3C/svg%3E");
  pointer-events: none;
  z-index: -1;
}

.hero {
  display: grid;
  grid-template-columns: minmax(0, 1.1fr) minmax(0, 0.9fr);
  gap: 40px;
  padding: 60px 8vw 30px;
  align-items: center;
}

.hero-content h1 {
  font-family: "Fraunces", serif;
  font-weight: 700;
  font-size: clamp(2.8rem, 4vw, 4rem);
  margin: 12px 0 16px;
}

.eyebrow {
  text-transform: uppercase;
  letter-spacing: 0.28em;
  font-size: 0.78rem;
  color: var(--cool);
}

.subhead {
  font-size: 1.1rem;
  max-width: 520px;
  color: var(--muted);
}

.hero-actions {
  display: flex;
  gap: 16px;
  margin-top: 28px;
}

.primary,
.ghost {
  border: none;
  padding: 12px 20px;
  font-size: 1rem;
  font-weight: 600;
  cursor: pointer;
  border-radius: 999px;
  transition: transform 0.2s ease, box-shadow 0.2s ease;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  text-decoration: none;
}

.primary {
  background: linear-gradient(135deg, var(--accent), var(--accent-dark));
  color: #fff;
  box-shadow: 0 16px 30px -20px rgba(214, 107, 45, 0.6);
}

.ghost {
  background: transparent;
  border: 1px solid var(--stroke);
  color: var(--ink);
}

.ghost.small {
  padding: 8px 14px;
  font-size: 0.9rem;
}

.toggle {
  border: 1px solid var(--stroke);
  background: var(--bg-deep);
  padding: 6px 14px;
  border-radius: 999px;
  font-weight: 600;
  cursor: pointer;
}

.primary:hover,
.ghost:hover {
  transform: translateY(-2px);
  box-shadow: 0 12px 24px -20px rgba(0, 0, 0, 0.3);
}

.toggle:hover {
  transform: translateY(-1px);
}

.toggle.off {
  background: transparent;
}

.hero-panel {
  display: grid;
  gap: 20px;
}

.panel-card {
  background: var(--panel);
  padding: 24px;
  border-radius: 24px;
  border: 1px solid var(--stroke);
  box-shadow: var(--panel-shadow);
}

.panel-label {
  margin: 0 0 14px;
  font-size: 0.85rem;
  text-transform: uppercase;
  letter-spacing: 0.2em;
  color: var(--muted);
}

.panel-row {
  display: flex;
  justify-content: space-between;
  font-size: 1rem;
  padding: 8px 0;
  align-items: center;
}

.panel-card ul {
  margin: 0;
  padding-left: 18px;
  color: var(--muted);
}

.difficulty {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
}

.chip {
  border-radius: 999px;
  border: 1px solid var(--stroke);
  background: transparent;
  padding: 8px 14px;
  font-weight: 600;
  cursor: pointer;
}

.chip.small {
  padding: 6px 10px;
  font-size: 0.75rem;
  letter-spacing: 0.08em;
  text-transform: uppercase;
}

.chip.active {
  background: var(--cool);
  color: #fff;
  border-color: transparent;
}

.chip.locked,
.chip:disabled {
  opacity: 0.6;
  cursor: not-allowed;
  border-style: dashed;
}

.game {
  padding: 0 8vw 60px;
}

.chapter-intro {
  display: grid;
  gap: 22px;
  margin-bottom: 34px;
}

.intro-card {
  background: var(--panel);
  border-radius: 24px;
  border: 1px solid var(--stroke);
  padding: 22px;
  box-shadow: var(--panel-shadow);
}

.intro-card h3 {
  margin: 0 0 12px;
  font-size: 1.2rem;
}

.rule-list {
  margin: 0;
  padding-left: 18px;
  color: var(--muted);
}

.rule-list li {
  margin: 6px 0;
}

.mode-tabs {
  display: flex;
  flex-wrap: wrap;
  gap: 12px;
  margin-bottom: 28px;
}

.mode-tabs .chip {
  padding: 10px 16px;
}

.chart-grid {
  display: grid;
  gap: 20px;
  grid-template-columns: repeat(auto-fit, minmax(280px, 1fr));
  margin-bottom: 28px;
}

.chart-card {
  background: var(--panel);
  border-radius: 24px;
  border: 1px solid var(--stroke);
  padding: 18px;
  box-shadow: var(--panel-shadow);
}

.chart-image {
  width: 100%;
  border-radius: 16px;
  border: 1px solid var(--stroke);
  background: #fff;
}

.chart-card .panel-label {
  margin-bottom: 12px;
}

.diagram-toolbar {
  display: flex;
  justify-content: flex-end;
  margin-bottom: 12px;
}

.overlap-active .mo-column {
  border-color: rgba(15, 61, 62, 0.35);
  box-shadow: 0 0 0 2px rgba(15, 61, 62, 0.08);
}

.overlap-active .mo-column h4 {
  color: var(--cool);
}

.mo-symbol {
  display: inline-block;
  font-weight: 700;
  color: var(--cool);
  margin-right: 6px;
}

.game-board {
  display: grid;
  grid-template-columns: minmax(0, 1.2fr) minmax(0, 0.8fr);
  gap: 32px;
  align-items: start;
}

.question-card,
.answer-card {
  background: var(--panel);
  border-radius: 28px;
  padding: 28px;
  border: 1px solid var(--stroke);
  box-shadow: var(--panel-shadow);
}

.question-header {
  display: flex;
  gap: 10px;
  margin-bottom: 16px;
}

.badge {
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.15em;
  background: var(--bg-deep);
  padding: 6px 10px;
  border-radius: 999px;
}

.badge.muted {
  background: transparent;
  border: 1px solid var(--stroke);
  color: var(--muted);
}

.question-card h2 {
  font-size: clamp(1.6rem, 2.6vw, 2.2rem);
  margin: 0 0 12px;
}

.question-note {
  color: var(--muted);
  margin-bottom: 20px;
}

.diagram {
  position: relative;
  width: min(320px, 60vw);
  aspect-ratio: 1;
  margin: 0 auto;
  border-radius: 24px;
  border: 1px dashed var(--stroke);
  background: radial-gradient(circle, #fff, var(--bg));
}

.diagram-actions {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 12px;
  margin-top: 18px;
}

.diagram-label {
  font-size: 0.85rem;
  color: var(--muted);
}

.viewer-card {
  margin-top: 18px;
  padding: 16px;
  border-radius: 20px;
  border: 1px solid var(--stroke);
  background: #fff;
}

.viewer-header {
  display: flex;
  align-items: center;
  justify-content: space-between;
  margin-bottom: 10px;
}

.viewer-controls {
  display: flex;
  gap: 8px;
}

.viewer-label {
  font-size: 0.8rem;
  color: var(--muted);
  letter-spacing: 0.12em;
  text-transform: uppercase;
}

.viewer3d {
  width: 100%;
  height: 260px;
  border-radius: 16px;
  border: 1px solid var(--stroke);
  background: radial-gradient(circle at top, #ffffff, var(--bg));
}

.viewer-note {
  font-size: 0.85rem;
  color: var(--muted);
  margin: 10px 0 0;
}

.central {
  position: absolute;
  inset: 0;
  margin: auto;
  width: 74px;
  height: 74px;
  border-radius: 50%;
  background: var(--cool);
  color: #fff;
  display: grid;
  place-items: center;
  font-weight: 600;
  font-size: 1.2rem;
  box-shadow: inset 0 0 12px rgba(255, 255, 255, 0.2);
}

.ligand {
  position: absolute;
  width: 70px;
  padding: 6px 8px;
  border-radius: 16px;
  background: #fff;
  border: 1px solid var(--stroke);
  text-align: center;
  font-size: 0.9rem;
  box-shadow: 0 10px 20px -16px rgba(0, 0, 0, 0.35);
}

.ligand.top {
  top: 18px;
  left: 50%;
  transform: translateX(-50%);
}

.ligand.bottom {
  bottom: 18px;
  left: 50%;
  transform: translateX(-50%);
}

.ligand.left {
  left: 14px;
  top: 50%;
  transform: translateY(-50%);
}

.ligand.right {
  right: 14px;
  top: 50%;
  transform: translateY(-50%);
}

.ligand.front {
  left: 24px;
  top: 24px;
  background: #fef3e9;
}

.ligand.back {
  right: 24px;
  bottom: 24px;
  background: #eef6f6;
}

.answer-label {
  font-size: 0.85rem;
  text-transform: uppercase;
  letter-spacing: 0.16em;
  color: var(--muted);
}

#answerInput {
  width: 100%;
  padding: 14px 16px;
  margin: 12px 0 16px;
  border-radius: 16px;
  border: 1px solid var(--stroke);
  font-size: 1rem;
  font-family: inherit;
}

.answer-actions {
  display: flex;
  gap: 12px;
}

.feedback {
  margin-top: 14px;
  padding: 12px 14px;
  border-radius: 16px;
  background: #f7f6f2;
  color: var(--muted);
  min-height: 54px;
}

.feedback.success {
  background: #e9f7f0;
  color: #215c3b;
}

.feedback.error {
  background: #fff1f0;
  color: #8a1f15;
}

.progress {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  margin-top: 20px;
  gap: 8px;
  text-align: center;
  font-size: 0.9rem;
  color: var(--muted);
}

.progress-label {
  display: block;
  font-size: 0.75rem;
  letter-spacing: 0.14em;
  text-transform: uppercase;
  margin-bottom: 4px;
}

.answer-block {
  margin-bottom: 18px;
}

.answer-block h4 {
  margin: 0 0 10px;
  font-size: 1rem;
}

.option-grid {
  display: grid;
  gap: 10px;
  grid-template-columns: repeat(auto-fit, minmax(120px, 1fr));
}

.option-button {
  border-radius: 16px;
  border: 1px solid var(--stroke);
  background: transparent;
  padding: 10px 12px;
  cursor: pointer;
  font-weight: 600;
  color: var(--ink);
}

.option-button.active {
  background: var(--cool);
  color: #fff;
  border-color: transparent;
}

.option-button.correct {
  background: #e9f7f0;
  color: #215c3b;
  border-color: transparent;
}

.option-button.wrong {
  background: #fff1f0;
  color: #8a1f15;
  border-color: transparent;
}

.form-row {
  display: grid;
  gap: 10px;
  grid-template-columns: repeat(auto-fit, minmax(140px, 1fr));
}

.form-row select {
  padding: 10px 12px;
  border-radius: 14px;
  border: 1px solid var(--stroke);
  font-family: inherit;
}

.hint-chip {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  padding: 6px 12px;
  border-radius: 999px;
  border: 1px dashed var(--stroke);
  color: var(--muted);
  font-size: 0.8rem;
}

.orbital-grid {
  display: grid;
  gap: 12px;
  margin-top: 14px;
}

.orbital-frame {
  display: grid;
  grid-template-columns: 36px 1fr;
  gap: 14px;
  align-items: start;
}

.energy-axis {
  position: relative;
  height: 100%;
  min-height: 220px;
  border-left: 2px solid var(--stroke);
}

.energy-axis::before {
  content: "high";
  position: absolute;
  top: -10px;
  left: -8px;
  transform: rotate(-90deg);
  font-size: 0.7rem;
  color: var(--muted);
}

.energy-axis::after {
  content: "low";
  position: absolute;
  bottom: -4px;
  left: -6px;
  transform: rotate(-90deg);
  font-size: 0.7rem;
  color: var(--muted);
}

.orbital-row {
  display: grid;
  grid-template-columns: 120px 1fr;
  gap: 12px;
  align-items: center;
}

.orbital-label {
  font-weight: 600;
  color: var(--muted);
}

.orbital-slots {
  display: flex;
  gap: 10px;
}

.orbital-slot {
  width: 54px;
  height: 40px;
  border-radius: 12px;
  border: 1px solid var(--stroke);
  display: grid;
  place-items: center;
  background: #fff;
}

.orbital-slot[data-spin="up"]::after {
  content: "U";
  color: rgba(28, 27, 26, 0.3);
  font-weight: 700;
}

.orbital-slot[data-spin="down"]::after {
  content: "D";
  color: rgba(28, 27, 26, 0.3);
  font-weight: 700;
}

.orbital-slot.active {
  border-color: var(--accent);
  box-shadow: 0 0 0 2px rgba(214, 107, 45, 0.2);
}

.electron-bank {
  margin-top: 18px;
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
  padding: 12px;
  border-radius: 16px;
  border: 1px dashed var(--stroke);
  background: #fff;
}

.cfse-panel {
  margin-top: 14px;
  display: grid;
  gap: 6px;
  font-size: 0.9rem;
  color: var(--muted);
}

.cfse-panel strong {
  color: var(--ink);
}

.electron-token {
  width: 34px;
  height: 34px;
  border-radius: 50%;
  background: linear-gradient(135deg, #fff1df, #f9d6c1);
  border: 1px solid rgba(214, 107, 45, 0.4);
  display: grid;
  place-items: center;
  font-weight: 700;
  cursor: grab;
}

.electron-token:active {
  cursor: grabbing;
}

.mo-builder {
  display: grid;
  gap: 14px;
  margin-top: 12px;
}

.mo-diagram {
  display: grid;
  grid-template-columns: 1fr 1fr 1.4fr;
  gap: 16px;
  align-items: start;
}

.mo-column {
  border-radius: 16px;
  border: 1px solid var(--stroke);
  padding: 12px;
  background: #fff;
}

.mo-column.overlap {
  position: relative;
  overflow: hidden;
}

.mo-bands {
  position: absolute;
  inset: 0;
  pointer-events: none;
}

.mo-band {
  position: absolute;
  left: 8px;
  right: 8px;
  height: 26%;
  border-radius: 12px;
  display: flex;
  align-items: center;
  padding-left: 10px;
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.12em;
  color: rgba(28, 27, 26, 0.6);
}

.mo-band.sigma {
  top: 6%;
  background: rgba(15, 61, 62, 0.08);
  border: 1px solid rgba(15, 61, 62, 0.18);
}

.mo-band.pi {
  top: 38%;
  background: rgba(214, 107, 45, 0.08);
  border: 1px solid rgba(214, 107, 45, 0.18);
}

.mo-column h4 {
  margin: 0 0 10px;
  font-size: 0.95rem;
  color: var(--muted);
  text-transform: uppercase;
  letter-spacing: 0.12em;
}

.mo-list {
  display: grid;
  gap: 8px;
  font-size: 0.9rem;
  color: var(--ink);
}

.mo-level-label {
  font-size: 0.85rem;
  color: var(--muted);
  margin-left: 8px;
}

.mo-targets {
  display: grid;
  gap: 12px;
}

.mo-target {
  border-radius: 16px;
  border: 1px dashed var(--stroke);
  padding: 10px 12px;
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.mo-target span {
  color: var(--muted);
}

.mo-bank {
  display: flex;
  gap: 12px;
  flex-wrap: wrap;
}

.mo-card {
  padding: 8px 14px;
  border-radius: 999px;
  border: 1px solid var(--stroke);
  background: #fff;
  cursor: grab;
  font-weight: 600;
}

.info {
  margin-top: 40px;
  display: grid;
  gap: 18px;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
}

.info-card {
  padding: 22px;
  border-radius: 20px;
  border: 1px solid var(--stroke);
  background: #fff;
  color: var(--muted);
}

.footer {
  text-align: center;
  padding: 32px;
  font-size: 0.85rem;
  color: var(--muted);
}

@media (max-width: 980px) {
  .hero {
    grid-template-columns: 1fr;
  }

  .game-board {
    grid-template-columns: 1fr;
  }

  .hero-actions {
    flex-direction: column;
    align-items: flex-start;
  }
}