* {
  box-sizing: border-box;
}

:root {
  --bg: #f6f3ea;
  --bg-alt: #ece7d9;
  --ink: #1c1b18;
  --ink-soft: #4a463f;
  --line: #1c1b18;
}

body {
  margin: 0;
  font-family: Georgia, "Times New Roman", serif;
  background: var(--bg);
  color: var(--ink);
  line-height: 1.6;
}

header {
  border-bottom: 2px solid var(--line);
  padding: 2rem 1rem 1rem;
  text-align: center;
  background: var(--bg-alt);
}

header h1 {
  margin: 0;
  letter-spacing: 0.05em;
  text-transform: uppercase;
}

.subtitle {
  margin: 0.3rem 0 0;
  font-style: italic;
  color: var(--ink-soft);
}

nav {
  display: flex;
  justify-content: center;
  flex-wrap: wrap;
  gap: 1.2rem;
  padding: 0.75rem;
  border-bottom: 1px solid var(--line);
  background: var(--bg);
  position: sticky;
  top: 0;
}

nav a {
  color: var(--ink);
  text-decoration: none;
  border-bottom: 1px solid transparent;
  font-size: 0.95rem;
}

nav a:hover {
  border-bottom: 1px solid var(--ink);
}

main {
  max-width: 860px;
  margin: 0 auto;
  padding: 1.5rem 1rem 3rem;
}

section {
  margin-bottom: 3.5rem;
}

h2 {
  border-bottom: 1px solid var(--line);
  padding-bottom: 0.3rem;
}

table {
  width: 100%;
  border-collapse: collapse;
  margin: 1rem 0;
}

th, td {
  border: 1px solid var(--line);
  padding: 0.5rem 0.7rem;
  text-align: left;
}

th {
  background: var(--ink);
  color: var(--bg);
}

tr:nth-child(even) {
  background: var(--bg-alt);
}

select, input[type="text"] {
  border: 1px solid var(--line);
  background: var(--bg);
  color: var(--ink);
  padding: 0.4rem;
  font-family: inherit;
  margin-right: 0.5rem;
}

button {
  background: var(--ink);
  color: var(--bg);
  border: 1px solid var(--ink);
  padding: 0.4rem 1rem;
  cursor: pointer;
  font-family: inherit;
  margin-right: 0.5rem;
}

button:hover {
  background: var(--bg);
  color: var(--ink);
}

#quiz-feedback {
  font-weight: bold;
  min-height: 1.5rem;
}

footer {
  border-top: 1px solid var(--line);
  text-align: center;
  padding: 1rem;
  font-size: 0.85rem;
  color: var(--ink-soft);
}

/* --- molecule viewer / builder --- */

.builder-controls {
  display: flex;
  flex-wrap: wrap;
  gap: 1rem;
  margin-bottom: 1rem;
}

.builder-controls label {
  display: flex;
  flex-direction: column;
  font-size: 0.85rem;
  color: var(--ink-soft);
}

.viewer-stage {
  border: 1px solid var(--line);
  background: var(--bg-alt);
  height: 280px;
  display: flex;
  align-items: center;
  justify-content: center;
  perspective: 700px;
  overflow: hidden;
}

.viewer-molecule {
  width: 220px;
  height: 220px;
  animation: spin 9s linear infinite;
  transform-style: preserve-3d;
}

.viewer-molecule svg {
  width: 100%;
  height: 100%;
}

@keyframes spin {
  from { transform: rotateY(0deg) rotateX(8deg); }
  to   { transform: rotateY(360deg) rotateX(8deg); }
}

.viewer-name {
  text-align: center;
  font-style: italic;
  color: var(--ink-soft);
  margin-top: 0.5rem;
}

.atom-label {
  font-size: 14px;
  font-family: Georgia, serif;
  fill: var(--ink);
  text-anchor: middle;
  dominant-baseline: middle;
}

.bond-line {
  stroke: var(--ink);
  stroke-width: 2.5;
}