:root {
  --bg-primary: #0f172a;
  --bg-card: #1e293b;
  --text-main: #e2e8f0;
  --text-muted: #94a3b8;
  --accent: #38bdf8;
  --accent-hover: #7dd3fc;
  --success: #22c55e;
  --error: #ef4444;
  --border-radius: 10px;
  --transition: 0.2s ease;
}

* { box-sizing: border-box; margin: 0; padding: 0; }
body {
  font-family: system-ui, -apple-system, sans-serif;
  background: var(--bg-primary);
  color: var(--text-main);
  line-height: 1.6;
  padding: 2rem 1rem;
}

.activity-header { text-align: center; margin-bottom: 2rem; }
.activity-header h1 { font-size: 1.8rem; margin-bottom: 0.5rem; }
.activity-header p { color: var(--text-muted); max-width: 600px; margin: 0 auto; }

.activity-container {
  max-width: 900px;
  margin: 0 auto;
}

/* Timeline Navigation */
.timeline-nav {
  display: flex;
  flex-wrap: wrap;
  gap: 0.75rem;
  justify-content: center;
  margin-bottom: 2rem;
}

.era-btn {
  background: var(--bg-card);
  border: 1px solid #334155;
  color: var(--text-main);
  padding: 0.6rem 1rem;
  border-radius: var(--border-radius);
  cursor: pointer;
  font-size: 0.95rem;
  transition: var(--transition);
}

.era-btn:hover, .era-btn.active {
  background: var(--accent);
  color: #0f172a;
  border-color: var(--accent);
  transform: translateY(-2px);
}

/* Detail Panel */
.detail-panel {
  background: var(--bg-card);
  padding: 1.5rem;
  border-radius: var(--border-radius);
  margin-bottom: 2rem;
  min-height: 200px;
}

.detail-panel h2 { color: var(--accent); margin-bottom: 0.75rem; }
.detail-panel .meta { display: flex; gap: 1rem; flex-wrap: wrap; margin-bottom: 1rem; font-size: 0.9rem; color: var(--text-muted); }
.detail-panel .meta span { background: #334155; padding: 0.2rem 0.6rem; border-radius: 6px; }
.detail-panel h3 { margin-top: 1rem; margin-bottom: 0.4rem; color: var(--accent-hover); font-size: 1.1rem; }
.detail-panel p, .detail-panel ul { margin-bottom: 0.8rem; }
.detail-panel ul { padding-left: 1.2rem; }

/* Quiz Section */
.quiz-section { background: var(--bg-card); padding: 1.5rem; border-radius: var(--border-radius); }
.quiz-question { margin-bottom: 1.5rem; padding-bottom: 1rem; border-bottom: 1px solid #334155; }
.quiz-options { display: flex; flex-direction: column; gap: 0.5rem; margin-top: 0.5rem; }
.option-btn {
  background: #334155;
  border: none;
  color: var(--text-main);
  padding: 0.7rem 1rem;
  text-align: left;
  border-radius: 6px;
  cursor: pointer;
  transition: var(--transition);
}
.option-btn:hover { background: #475569; }
.option-btn.correct { background: var(--success); color: #0f172a; }
.option-btn.incorrect { background: var(--error); }
.feedback { margin-top: 0.5rem; font-size: 0.9rem; min-height: 1.4em; }

.btn-secondary {
  background: transparent;
  border: 1px solid var(--accent);
  color: var(--accent);
  padding: 0.6rem 1.2rem;
  border-radius: 6px;
  cursor: pointer;
  margin-top: 1rem;
}
.btn-secondary:hover { background: rgba(56, 189, 248, 0.1); }

/* Responsive */
@media (max-width: 600px) {
  .timeline-nav { gap: 0.5rem; }
  .era-btn { font-size: 0.85rem; padding: 0.5rem 0.7rem; }
}