.intro-card {
  border-radius: var(--radius-lg);
  padding: 22px 24px;
  margin-bottom: 24px;
  line-height: 1.7;
}

.intro-card p {
  margin: 0 0 10px;
}

.intro-card p:last-child {
  margin-bottom: 0;
}

.intro-note {
  color: var(--muted);
}

.lesson-layout {
  grid-template-columns: minmax(0, 1.6fr) minmax(260px, 0.8fr);
  margin-bottom: 24px;
}

.explorer-card,
.legend-card,
.table-section,
.cta-section {
  border-radius: var(--radius-lg);
  padding: 24px;
}

/* ── Root picker ───────────────────────────────────────────── */
.root-picker {
  margin-top: 18px;
}

.root-picker-label {
  display: block;
  font-size: 0.75rem;
  text-transform: uppercase;
  letter-spacing: 0.1em;
  color: var(--muted);
  margin-bottom: 10px;
}

.root-picker-buttons {
  display: grid;
  grid-template-columns: repeat(12, minmax(0, 1fr));
  gap: 6px;
}

.root-btn {
  border: 1px solid rgba(255, 255, 255, 0.1);
  background: var(--surface-soft);
  color: var(--text);
  border-radius: 10px;
  padding: 9px 0;
  cursor: pointer;
  font-weight: 700;
  font-size: 0.85rem;
  transition: background 140ms ease, border-color 140ms ease, transform 120ms ease;
}

.root-btn:hover,
.root-btn:focus-visible {
  transform: translateY(-1px);
  background: rgba(255, 255, 255, 0.08);
  outline: none;
}

.root-btn.is-active {
  background: linear-gradient(180deg, var(--accent) 0%, var(--accent-strong) 100%);
  border-color: transparent;
  color: #0b1220;
}

/* ── Step buttons (0-12) ──────────────────────────────────── */
.step-buttons {
  display: grid;
  grid-template-columns: repeat(13, minmax(0, 1fr));
  gap: 6px;
  margin-top: 12px;
}

.step-btn {
  border: 1px solid rgba(255, 255, 255, 0.1);
  background: var(--surface-soft);
  color: var(--text);
  border-radius: 10px;
  padding: 10px 0;
  cursor: pointer;
  font-weight: 700;
  transition: background 140ms ease, border-color 140ms ease, transform 120ms ease;
}

.step-btn:hover,
.step-btn:focus-visible {
  transform: translateY(-1px);
  background: rgba(255, 255, 255, 0.08);
  outline: none;
}

.step-btn.is-active {
  background: linear-gradient(180deg, var(--danger-strong) 0%, var(--danger) 100%);
  border-color: transparent;
}

/* ── Interval info panel ──────────────────────────────────── */
.interval-info {
  margin-top: 20px;
  padding: 18px 20px;
  border-radius: var(--radius-md);
  background: rgba(255, 255, 255, 0.04);
  border: 1px solid rgba(255, 255, 255, 0.08);
}

.interval-info-head {
  display: flex;
  align-items: center;
  gap: 12px;
}

.interval-info-head h3 {
  margin: 0;
  font-size: 1.3rem;
}

.interval-notes {
  margin: 10px 0 0;
  color: var(--muted);
  font-weight: 600;
}

.interval-mnemonic {
  margin: 6px 0 0;
}

.interval-tip {
  margin: 6px 0 0;
  color: var(--muted);
  font-size: 0.9rem;
}

/* ── Legend card ───────────────────────────────────────────── */
.legend-heading {
  margin: 4px 0 16px;
}

.legend-list {
  list-style: none;
  margin: 0;
  padding: 0;
  display: flex;
  flex-direction: column;
  gap: 12px;
}

.legend-list li {
  display: flex;
  align-items: flex-start;
  gap: 10px;
  line-height: 1.5;
  color: var(--muted);
  font-size: 0.92rem;
}

.legend-list .interval-chip {
  flex-shrink: 0;
}

.legend-tip-kicker {
  margin-top: 24px;
}

.legend-tip {
  margin: 4px 0 0;
  line-height: 1.6;
  color: var(--muted);
}

/* ── CTA ───────────────────────────────────────────────────── */
.cta-section {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 20px;
  flex-wrap: wrap;
}

.cta-section h2 {
  margin: 4px 0 0;
}

.cta-copy {
  margin: 8px 0 0;
  color: var(--muted);
  max-width: 46ch;
}

.cta-button {
  flex-shrink: 0;
  background: linear-gradient(180deg, var(--accent) 0%, var(--accent-strong) 100%);
  color: #0b1220;
  text-decoration: none;
  font-weight: 800;
  padding: 16px 26px;
  border-radius: var(--radius-sm);
  box-shadow: 0 14px 30px rgba(78, 204, 163, 0.24);
  transition: transform 140ms ease, box-shadow 140ms ease;
}

.cta-button:hover,
.cta-button:focus-visible {
  transform: translateY(-2px);
  box-shadow: 0 18px 36px rgba(78, 204, 163, 0.32);
  outline: none;
}

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

@media (max-width: 760px) {
  .root-picker-buttons {
    grid-template-columns: repeat(6, minmax(0, 1fr));
    gap: 6px;
  }

  .step-buttons {
    grid-template-columns: repeat(7, minmax(0, 1fr));
  }

  .cta-section {
    flex-direction: column;
    align-items: stretch;
    text-align: center;
  }
}