/* ── Main Content ── */
  .main {
    margin-left: 260px;
    padding: 40px 60px 100px;
    max-width: 1100px;
    box-sizing: border-box;
  }

  /* ── Hero Header ── */
  .hero {
    text-align: center;
    padding: 60px 0 40px;
    margin-bottom: 40px;
    border-bottom: 1px solid var(--border);
  }
  .hero h1 {
    font-size: 42px;
    font-weight: 900;
    background: linear-gradient(135deg, var(--concept), var(--formula));
    -webkit-background-clip: text;
    -webkit-text-fill-color: transparent;
    margin-bottom: 12px;
  }
  .hero p {
    color: var(--text-secondary);
    font-size: 16px;
  }

  /* ── Section Titles ── */
  .lecture-section {
    margin-bottom: 60px;
    scroll-margin-top: 30px;
  }
  .lecture-header {
    display: flex;
    align-items: center;
    gap: 16px;
    margin-bottom: 32px;
    padding-bottom: 16px;
    border-bottom: 2px solid var(--concept);
  }
  .lecture-num {
    background: linear-gradient(135deg, var(--concept), var(--concept-dark));
    color: white;
    width: 52px; height: 52px;
    border-radius: 14px;
    display: flex; align-items: center; justify-content: center;
    font-size: 22px;
    font-weight: 900;
    flex-shrink: 0;
  }
  .lecture-header h2 {
    font-size: 28px;
    font-weight: 700;
    color: var(--text-white);
  }