* { margin: 0; padding: 0; box-sizing: border-box; }

  :root {
    --paper:        #f4f1ea;
    --paper-soft:   #ebe7dd;
    --paper-line:   #d9d3c4;
    --ink:          #1a1a1a;
    --ink-soft:     #4a4a48;
    --ink-mute:     #7a766c;
    --rule:         #cdc6b6;

    --accent:       #7a2a2a;
    --accent-soft:  #b56666;
    --accent-bg:    #ecdada;

    --highlight:    #e8d8a4;
    --paper-warm:   #f8eccb;

    --green:        #3d6b4a;
    --blue:         #1f3a5f;
    --amber:        #8a5a1a;

    --font-serif:   'Fraunces', 'Iowan Old Style', Georgia, serif;
    --font-sans:    'Inter', -apple-system, BlinkMacSystemFont, sans-serif;
    --font-mono:    'JetBrains Mono', 'SF Mono', Menlo, monospace;
  }

  html { scroll-behavior: smooth; }

  body {
    font-family: var(--font-sans);
    background: var(--paper);
    color: var(--ink);
    line-height: 1.6;
    font-size: 16px;
    -webkit-font-smoothing: antialiased;
    text-rendering: optimizeLegibility;
    background-image:
      radial-gradient(circle at 20% 12%, rgba(122, 42, 42, 0.025) 0%, transparent 35%),
      radial-gradient(circle at 80% 65%, rgba(31, 58, 95, 0.02) 0%, transparent 40%);
  }

  /* Subtle paper texture using SVG noise */
  body::before {
    content: '';
    position: fixed;
    inset: 0;
    pointer-events: none;
    z-index: 0;
    opacity: 0.4;
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='200' height='200'><filter id='n'><feTurbulence type='fractalNoise' baseFrequency='0.85' numOctaves='2' stitchTiles='stitch'/><feColorMatrix values='0 0 0 0 0.1  0 0 0 0 0.1  0 0 0 0 0.05  0 0 0 0.04 0'/></filter><rect width='100%25' height='100%25' filter='url(%23n)'/></svg>");
  }

  ::selection { background: var(--highlight); color: var(--ink); }

  /* ============ NAV ============ */
  nav {
    position: sticky;
    top: 0;
    background: rgba(244, 241, 234, 0.92);
    backdrop-filter: saturate(180%) blur(10px);
    -webkit-backdrop-filter: saturate(180%) blur(10px);
    border-bottom: 1px solid var(--rule);
    z-index: 50;
  }
  .nav-inner {
    max-width: 1180px;
    margin: 0 auto;
    padding: 18px 32px;
    display: flex;
    align-items: center;
    justify-content: space-between;
    gap: 32px;
  }
  .brand {
    display: flex;
    align-items: center;
    gap: 12px;
    font-family: var(--font-serif);
    font-weight: 600;
    font-size: 1.15rem;
    letter-spacing: -0.01em;
    color: var(--ink);
  }
  .brand-mark {
    width: 32px; height: 32px;
    border: 1.5px solid var(--ink);
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    background: var(--paper);
  }
  .brand-mark svg { width: 18px; height: 18px; }
  .brand small {
    font-family: var(--font-sans);
    font-weight: 400;
    color: var(--ink-mute);
    font-size: 0.78rem;
    letter-spacing: 0.05em;
    text-transform: uppercase;
    margin-left: 8px;
  }

  .nav-links {
    list-style: none;
    display: flex;
    gap: 4px;
    align-items: center;
    flex-wrap: wrap;
  }
  .nav-links a {
    color: var(--ink-soft);
    text-decoration: none;
    font-size: 0.9rem;
    font-weight: 500;
    padding: 8px 14px;
    border-radius: 4px;
    transition: color 0.2s;
    position: relative;
  }
  .nav-links a:hover { color: var(--ink); }
  .nav-links a.active {
    color: var(--accent);
  }
  .nav-links a.active::after {
    content: '';
    position: absolute;
    left: 14px; right: 14px; bottom: 4px;
    height: 1.5px;
    background: var(--accent);
  }

  /* ============ LAYOUT ============ */
  .container {
    max-width: 1180px;
    margin: 0 auto;
    padding: 0 32px;
    position: relative;
    z-index: 1;
  }
  .container-narrow {
    max-width: 820px;
    margin: 0 auto;
    padding: 0 32px;
    position: relative;
    z-index: 1;
  }

  section { padding: 96px 0; position: relative; z-index: 1; }
  section + section { border-top: 1px solid var(--rule); }

  /* ============ HERO ============ */
  .hero {
    padding: 120px 0 100px;
    position: relative;
  }
  .hero-grid {
    display: grid;
    grid-template-columns: 1.4fr 1fr;
    gap: 80px;
    align-items: center;
  }
  @media (max-width: 900px) {
    .hero-grid { grid-template-columns: 1fr; gap: 48px; }
  }

  .eyebrow {
    display: inline-flex;
    align-items: center;
    gap: 10px;
    font-size: 0.78rem;
    text-transform: uppercase;
    letter-spacing: 0.18em;
    color: var(--ink-mute);
    font-weight: 500;
    margin-bottom: 28px;
  }
  .eyebrow .rule {
    width: 32px; height: 1px;
    background: var(--ink-mute);
  }

  .hero h1 {
    font-family: var(--font-serif);
    font-weight: 400;
    font-size: clamp(3rem, 7vw, 5.5rem);
    line-height: 1.02;
    letter-spacing: -0.025em;
    margin-bottom: 28px;
  }
  .hero h1 em {
    font-style: italic;
    font-weight: 300;
    color: var(--accent);
  }
  .hero-lead {
    font-size: 1.18rem;
    color: var(--ink-soft);
    line-height: 1.65;
    max-width: 540px;
    margin-bottom: 40px;
    font-weight: 400;
  }
  .hero-lead strong { color: var(--ink); font-weight: 600; }

  .hero-actions { display: flex; gap: 14px; flex-wrap: wrap; align-items: center; }

  .btn {
    display: inline-flex;
    align-items: center;
    gap: 8px;
    padding: 13px 22px;
    font-size: 0.92rem;
    font-weight: 500;
    font-family: var(--font-sans);
    text-decoration: none;
    border: 1px solid transparent;
    cursor: pointer;
    transition: all 0.2s ease;
    border-radius: 2px;
  }
  .btn svg { width: 14px; height: 14px; }
  .btn-primary {
    background: var(--ink);
    color: var(--paper);
    border-color: var(--ink);
  }
  .btn-primary:hover { background: var(--accent); border-color: var(--accent); }
  .btn-ghost {
    background: transparent;
    color: var(--ink);
    border-color: var(--ink);
  }
  .btn-ghost:hover { background: var(--ink); color: var(--paper); }

  /* Hero illustration: scientific atomic diagram */
  .hero-illo {
    position: relative;
    aspect-ratio: 1;
    max-width: 420px;
    margin-left: auto;
  }
  .hero-illo svg {
    width: 100%;
    height: 100%;
    display: block;
  }
  .hero-illo .illo-frame {
    position: absolute;
    inset: 0;
    border: 1px solid var(--rule);
    pointer-events: none;
  }
  .hero-illo .illo-corner {
    position: absolute;
    width: 12px; height: 12px;
    border: 1px solid var(--ink);
    background: var(--paper);
  }
  .illo-corner.tl { top: -6px; left: -6px; }
  .illo-corner.tr { top: -6px; right: -6px; }
  .illo-corner.bl { bottom: -6px; left: -6px; }
  .illo-corner.br { bottom: -6px; right: -6px; }
  .illo-caption {
    position: absolute;
    bottom: -36px;
    left: 0; right: 0;
    text-align: center;
    font-family: var(--font-mono);
    font-size: 0.7rem;
    letter-spacing: 0.15em;
    color: var(--ink-mute);
    text-transform: uppercase;
  }

  /* ============ SECTION HEADER ============ */
  .section-header {
    margin-bottom: 56px;
    max-width: 720px;
  }
  .section-num {
    font-family: var(--font-mono);
    font-size: 0.78rem;
    color: var(--accent);
    letter-spacing: 0.15em;
    margin-bottom: 16px;
    font-weight: 500;
  }
  .section-title {
    font-family: var(--font-serif);
    font-size: clamp(2.2rem, 4.5vw, 3.4rem);
    line-height: 1.05;
    letter-spacing: -0.02em;
    font-weight: 400;
    margin-bottom: 18px;
  }
  .section-title em { font-style: italic; color: var(--accent); }
  .section-lead {
    font-size: 1.08rem;
    color: var(--ink-soft);
    line-height: 1.65;
    max-width: 640px;
  }

  /* ============ DEFINITIONS / CARDS ============ */
  .def-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 0;
    border-top: 1px solid var(--ink);
    border-bottom: 1px solid var(--ink);
  }
  @media (max-width: 800px) {
    .def-grid { grid-template-columns: 1fr; }
    .def-item + .def-item { border-top: 1px solid var(--rule); border-left: none !important; }
  }
  .def-item {
    padding: 36px 28px;
    border-left: 1px solid var(--rule);
    transition: background 0.3s;
    position: relative;
  }
  .def-item:first-child { border-left: none; }
  .def-item:hover { background: var(--paper-soft); }

  .def-icon {
    width: 36px; height: 36px;
    margin-bottom: 22px;
    color: var(--accent);
  }
  .def-icon svg { width: 100%; height: 100%; }
  .def-label {
    font-family: var(--font-mono);
    font-size: 0.7rem;
    letter-spacing: 0.15em;
    color: var(--ink-mute);
    text-transform: uppercase;
    margin-bottom: 8px;
  }
  .def-title {
    font-family: var(--font-serif);
    font-size: 1.55rem;
    font-weight: 500;
    margin-bottom: 14px;
    letter-spacing: -0.01em;
  }
  .def-text {
    color: var(--ink-soft);
    font-size: 0.95rem;
    line-height: 1.6;
    margin-bottom: 18px;
  }
  .def-list {
    list-style: none;
    padding: 0;
  }
  .def-list li {
    padding: 6px 0;
    color: var(--ink);
    font-size: 0.92rem;
    display: flex;
    align-items: center;
    gap: 10px;
    border-top: 1px dotted var(--rule);
  }
  .def-list li:first-child { border-top: none; }
  .def-list li::before {
    content: '';
    width: 5px; height: 5px;
    background: var(--accent);
    border-radius: 50%;
    flex-shrink: 0;
  }

  /* ============ CONVERTER PANEL ============ */
  .panel {
    background: var(--paper-soft);
    border: 1px solid var(--rule);
    border-radius: 2px;
    padding: 40px 36px;
    position: relative;
  }
  .panel::before {
    content: '';
    position: absolute;
    top: -1px; left: -1px; right: -1px;
    height: 3px;
    background: var(--ink);
  }
  .panel-header {
    display: flex;
    align-items: baseline;
    justify-content: space-between;
    gap: 20px;
    margin-bottom: 30px;
    padding-bottom: 18px;
    border-bottom: 1px solid var(--rule);
  }
  .panel-title {
    font-family: var(--font-serif);
    font-size: 1.5rem;
    font-weight: 500;
  }
  .panel-tag {
    font-family: var(--font-mono);
    font-size: 0.72rem;
    letter-spacing: 0.15em;
    color: var(--ink-mute);
    text-transform: uppercase;
  }

  .converter-row {
    display: grid;
    grid-template-columns: 1fr auto 1fr;
    gap: 18px;
    align-items: end;
  }
  @media (max-width: 720px) { .converter-row { grid-template-columns: 1fr; } }

  .input-group { display: flex; flex-direction: column; gap: 8px; }
  .input-label {
    font-family: var(--font-mono);
    font-size: 0.7rem;
    letter-spacing: 0.15em;
    color: var(--ink-mute);
    text-transform: uppercase;
  }
  .input-row { display: flex; gap: 8px; }

  .input-field, .select-field {
    flex: 1;
    background: var(--paper);
    border: 1px solid var(--rule);
    border-radius: 2px;
    padding: 12px 14px;
    color: var(--ink);
    font-size: 1rem;
    font-family: var(--font-sans);
    width: 100%;
    transition: border 0.2s, background 0.2s;
  }
  .input-field:focus, .select-field:focus {
    outline: none;
    border-color: var(--ink);
    background: #fff;
  }
  .select-field {
    cursor: pointer;
    appearance: none;
    -webkit-appearance: none;
    background-image: url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' fill='none' stroke='%237a766c' stroke-width='2'><polyline points='6 9 12 15 18 9'/></svg>");
    background-repeat: no-repeat;
    background-position: right 12px center;
    background-size: 14px;
    padding-right: 36px;
  }

  .swap-btn {
    align-self: flex-end;
    width: 44px; height: 44px;
    background: var(--paper);
    border: 1px solid var(--ink);
    border-radius: 2px;
    cursor: pointer;
    color: var(--ink);
    display: flex; align-items: center; justify-content: center;
    transition: all 0.25s;
  }
  .swap-btn:hover { background: var(--ink); color: var(--paper); transform: rotate(180deg); }
  .swap-btn svg { width: 18px; height: 18px; }

  .result-display {
    margin-top: 32px;
    padding: 28px 24px;
    background: var(--paper);
    border: 1px solid var(--ink);
    border-radius: 2px;
    text-align: center;
    position: relative;
  }
  .result-display::before {
    content: 'RESULTADO';
    position: absolute;
    top: -10px; left: 24px;
    background: var(--paper-soft);
    padding: 0 10px;
    font-family: var(--font-mono);
    font-size: 0.7rem;
    letter-spacing: 0.18em;
    color: var(--ink-mute);
  }
  .result-value {
    font-family: var(--font-serif);
    font-size: 2.6rem;
    font-weight: 400;
    color: var(--ink);
    letter-spacing: -0.02em;
    margin-bottom: 8px;
    line-height: 1.1;
    word-break: break-word;
  }
  .result-formula {
    color: var(--ink-mute);
    font-size: 0.95rem;
    line-height: 1.5;
  }
  .result-formula .katex { font-size: 1.05em; }

  /* ============ SCALE ============ */
  .scale-frame {
    margin: 36px 0;
    border: 1px solid var(--ink);
    background: var(--paper);
    overflow: hidden;
  }
  .scale-axis {
    display: grid;
    grid-template-columns: repeat(7, 1fr);
  }
  .scale-cell {
    padding: 22px 12px;
    text-align: center;
    border-right: 1px solid var(--rule);
    cursor: pointer;
    transition: background 0.25s;
    position: relative;
  }
  .scale-cell:last-child { border-right: none; }
  .scale-cell:hover { background: var(--paper-warm); }
  .scale-cell.unit {
    background: var(--ink);
    color: var(--paper);
  }
  .scale-cell.unit:hover { background: var(--accent); }

  .scale-prefix {
    font-family: var(--font-serif);
    font-size: 1.05rem;
    font-weight: 500;
    margin-bottom: 4px;
  }
  .scale-power {
    font-family: var(--font-mono);
    font-size: 0.78rem;
    color: var(--accent);
    margin-bottom: 4px;
  }
  .scale-cell.unit .scale-power { color: var(--paper); }
  .scale-value {
    font-family: var(--font-mono);
    font-size: 0.7rem;
    color: var(--ink-mute);
  }
  .scale-cell.unit .scale-value { color: rgba(244, 241, 234, 0.7); }

  .scale-legend {
    display: flex;
    justify-content: space-between;
    padding: 14px 20px;
    background: var(--paper-soft);
    border-top: 1px solid var(--rule);
    font-family: var(--font-mono);
    font-size: 0.72rem;
    letter-spacing: 0.12em;
    color: var(--ink-mute);
    text-transform: uppercase;
  }
  .scale-legend strong { color: var(--ink); }

  @media (max-width: 720px) {
    .scale-axis { grid-template-columns: repeat(4, 1fr); }
    .scale-cell { font-size: 0.85rem; padding: 14px 6px; }
  }

  /* ============ EXAMPLES ============ */
  .examples-grid {
    margin-top: 36px;
    border-top: 1px solid var(--ink);
  }
  .example-row {
    border-bottom: 1px solid var(--rule);
    transition: background 0.2s;
  }
  .example-row:hover { background: var(--paper-soft); }
  .example-row.open { background: var(--paper-soft); }

  .example-summary {
    display: flex;
    justify-content: space-between;
    align-items: center;
    padding: 22px 4px;
    cursor: pointer;
    user-select: none;
    gap: 24px;
  }
  .example-num {
    font-family: var(--font-mono);
    font-size: 0.78rem;
    color: var(--ink-mute);
    width: 32px;
    flex-shrink: 0;
  }
  .example-q {
    flex: 1;
    font-family: var(--font-serif);
    font-size: 1.15rem;
    font-weight: 500;
    color: var(--ink);
  }
  .example-q .katex { font-size: 1em; }
  .example-toggle {
    width: 32px; height: 32px;
    border: 1px solid var(--rule);
    border-radius: 2px;
    display: flex; align-items: center; justify-content: center;
    transition: all 0.25s;
    flex-shrink: 0;
    background: var(--paper);
  }
  .example-row.open .example-toggle {
    background: var(--ink);
    color: var(--paper);
    border-color: var(--ink);
    transform: rotate(45deg);
  }
  .example-toggle svg { width: 14px; height: 14px; }

  .example-detail {
    max-height: 0;
    overflow: hidden;
    transition: max-height 0.4s ease;
  }
  .example-row.open .example-detail { max-height: 500px; }
  .example-detail-inner {
    padding: 4px 0 26px 36px;
    border-left: 2px solid var(--accent);
    margin-left: 36px;
  }

  .step {
    padding: 10px 18px;
    background: var(--paper);
    border: 1px solid var(--rule);
    margin-bottom: 6px;
    font-family: var(--font-mono);
    font-size: 0.92rem;
    color: var(--ink-soft);
    display: flex;
    align-items: center;
    gap: 12px;
  }
  .step .step-tag {
    font-size: 0.65rem;
    letter-spacing: 0.15em;
    color: var(--ink-mute);
    text-transform: uppercase;
    padding: 2px 8px;
    background: var(--paper-soft);
    border-radius: 2px;
    flex-shrink: 0;
  }
  .step .katex { font-size: 1.05em; color: var(--ink); }

  .step-final {
    padding: 14px 20px;
    background: var(--ink);
    color: var(--paper);
    margin-top: 10px;
    font-family: var(--font-serif);
    font-size: 1.15rem;
    font-weight: 500;
    display: flex;
    align-items: center;
    gap: 12px;
  }
  .step-final .katex { color: var(--paper); }
  .step-final-tag {
    font-family: var(--font-mono);
    font-size: 0.7rem;
    letter-spacing: 0.18em;
    text-transform: uppercase;
    opacity: 0.65;
  }

  /* ============ TABS / TABLES ============ */
  .tabs {
    display: flex;
    border-bottom: 1px solid var(--ink);
    margin-bottom: 32px;
    flex-wrap: wrap;
  }
  .tab {
    background: transparent;
    border: none;
    color: var(--ink-mute);
    font-family: var(--font-sans);
    font-size: 0.95rem;
    font-weight: 500;
    padding: 14px 22px;
    cursor: pointer;
    position: relative;
    transition: color 0.2s;
    display: flex;
    align-items: center;
    gap: 10px;
    border-radius: 0;
  }
  .tab svg { width: 16px; height: 16px; }
  .tab:hover { color: var(--ink); }
  .tab.active { color: var(--ink); }
  .tab.active::after {
    content: '';
    position: absolute;
    left: 0; right: 0; bottom: -1.5px;
    height: 2.5px;
    background: var(--accent);
  }

  .tab-content { display: none; animation: fade .3s ease; }
  .tab-content.active { display: block; }

  @keyframes fade { from { opacity: 0; transform: translateY(4px); } to { opacity: 1; transform: translateY(0); } }

  .data-table {
    width: 100%;
    border-collapse: collapse;
    font-family: var(--font-sans);
    border-top: 1px solid var(--ink);
    border-bottom: 1px solid var(--ink);
  }
  .data-table th, .data-table td {
    padding: 16px 20px;
    text-align: left;
    border-bottom: 1px solid var(--rule);
  }
  .data-table tr:last-child td { border-bottom: none; }
  .data-table th {
    font-family: var(--font-mono);
    font-size: 0.72rem;
    letter-spacing: 0.15em;
    text-transform: uppercase;
    color: var(--ink-mute);
    font-weight: 500;
    background: var(--paper-soft);
  }
  .data-table tbody tr { transition: background 0.2s; }
  .data-table tbody tr:hover { background: var(--paper-soft); }
  .data-table .unit-name {
    font-family: var(--font-serif);
    font-size: 1.1rem;
    font-weight: 500;
  }
  .data-table .unit-symbol {
    font-family: var(--font-mono);
    font-size: 0.85rem;
    background: var(--paper);
    padding: 3px 8px;
    border: 1px solid var(--rule);
    border-radius: 2px;
    color: var(--accent);
    margin-left: 8px;
  }
  .data-table .unit-eq {
    font-family: var(--font-mono);
    color: var(--ink-soft);
  }
  .data-table .unit-eq .katex { font-size: 1.02em; color: var(--ink); }

  /* ============ TEMPERATURE ============ */
  .temp-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 0;
    border: 1px solid var(--ink);
    background: var(--paper);
  }
  @media (max-width: 800px) { .temp-grid { grid-template-columns: 1fr; } }

  .temp-card {
    padding: 32px 24px;
    text-align: center;
    border-right: 1px solid var(--rule);
    transition: background 0.25s;
    position: relative;
  }
  .temp-card:last-child { border-right: none; }
  @media (max-width: 800px) {
    .temp-card { border-right: none; border-bottom: 1px solid var(--rule); }
    .temp-card:last-child { border-bottom: none; }
  }
  .temp-card:focus-within { background: var(--paper-soft); }

  .temp-card-bar {
    height: 3px;
    width: 40px;
    margin: 0 auto 20px;
  }
  .temp-card.celsius .temp-card-bar { background: var(--blue); }
  .temp-card.fahrenheit .temp-card-bar { background: var(--amber); }
  .temp-card.kelvin .temp-card-bar { background: var(--accent); }

  .temp-name {
    font-family: var(--font-mono);
    font-size: 0.72rem;
    letter-spacing: 0.18em;
    text-transform: uppercase;
    color: var(--ink-mute);
    margin-bottom: 22px;
  }
  .temp-input {
    width: 100%;
    background: transparent;
    border: none;
    color: var(--ink);
    font-family: var(--font-serif);
    font-size: 3rem;
    font-weight: 400;
    text-align: center;
    outline: none;
    letter-spacing: -0.02em;
    -moz-appearance: textfield;
  }
  .temp-input::-webkit-outer-spin-button,
  .temp-input::-webkit-inner-spin-button {
    -webkit-appearance: none;
    margin: 0;
  }
  .temp-symbol {
    font-family: var(--font-serif);
    color: var(--ink-mute);
    font-size: 1rem;
    margin-top: 4px;
  }

  .temp-presets {
    display: flex;
    flex-wrap: wrap;
    gap: 8px;
    margin-top: 28px;
    justify-content: center;
  }
  .temp-preset {
    padding: 8px 16px;
    background: var(--paper);
    border: 1px solid var(--rule);
    color: var(--ink-soft);
    font-family: var(--font-sans);
    font-size: 0.88rem;
    cursor: pointer;
    transition: all 0.2s;
    border-radius: 2px;
  }
  .temp-preset:hover {
    background: var(--ink);
    color: var(--paper);
    border-color: var(--ink);
  }

  .formulas-block {
    margin-top: 36px;
    padding: 28px;
    background: var(--paper);
    border: 1px solid var(--rule);
  }
  .formulas-block-title {
    font-family: var(--font-mono);
    font-size: 0.72rem;
    letter-spacing: 0.18em;
    text-transform: uppercase;
    color: var(--ink-mute);
    margin-bottom: 18px;
  }
  .formulas-grid {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 14px;
  }
  @media (max-width: 600px) { .formulas-grid { grid-template-columns: 1fr; } }
  .formula-card {
    padding: 18px 22px;
    background: var(--paper-soft);
    border-left: 2px solid var(--accent);
    font-size: 1.05rem;
  }
  .formula-card .formula-label {
    font-family: var(--font-mono);
    font-size: 0.7rem;
    letter-spacing: 0.15em;
    text-transform: uppercase;
    color: var(--ink-mute);
    margin-bottom: 8px;
    display: block;
  }

  /* ============ QUIZ ============ */
  .quiz-card {
    background: var(--paper-soft);
    border: 1px solid var(--ink);
    padding: 40px 38px;
    max-width: 760px;
    margin: 0 auto;
    position: relative;
  }
  .quiz-card::before {
    content: '';
    position: absolute;
    top: -1px; left: -1px; right: -1px;
    height: 3px;
    background: var(--ink);
  }
  .quiz-progress {
    display: flex;
    gap: 6px;
    margin-bottom: 28px;
  }
  .quiz-dot {
    flex: 1;
    height: 4px;
    background: var(--rule);
    transition: background 0.3s;
  }
  .quiz-dot.active { background: var(--ink); }
  .quiz-dot.correct { background: var(--green); }
  .quiz-dot.wrong { background: var(--accent); }

  .quiz-meta {
    font-family: var(--font-mono);
    font-size: 0.72rem;
    letter-spacing: 0.15em;
    text-transform: uppercase;
    color: var(--ink-mute);
    margin-bottom: 14px;
  }
  .quiz-question {
    font-family: var(--font-serif);
    font-size: 1.5rem;
    font-weight: 500;
    line-height: 1.35;
    margin-bottom: 28px;
    letter-spacing: -0.01em;
  }
  .quiz-question .katex { font-size: 1em; }
  .quiz-options { display: grid; gap: 10px; }
  .quiz-option {
    background: var(--paper);
    border: 1px solid var(--rule);
    padding: 16px 20px;
    color: var(--ink);
    cursor: pointer;
    font-family: var(--font-sans);
    font-size: 1rem;
    text-align: left;
    display: flex;
    align-items: center;
    gap: 16px;
    transition: all 0.2s;
    border-radius: 2px;
  }
  .quiz-option:hover:not(:disabled) {
    border-color: var(--ink);
    background: var(--paper-warm);
  }
  .quiz-option-letter {
    width: 28px; height: 28px;
    border: 1px solid var(--rule);
    border-radius: 50%;
    display: flex; align-items: center; justify-content: center;
    font-family: var(--font-mono);
    font-size: 0.85rem;
    font-weight: 500;
    flex-shrink: 0;
    background: var(--paper-soft);
  }
  .quiz-option.correct {
    border-color: var(--green);
    background: rgba(61, 107, 74, 0.08);
    color: var(--green);
  }
  .quiz-option.correct .quiz-option-letter { background: var(--green); color: var(--paper); border-color: var(--green); }
  .quiz-option.wrong {
    border-color: var(--accent);
    background: var(--accent-bg);
    color: var(--accent);
  }
  .quiz-option.wrong .quiz-option-letter { background: var(--accent); color: var(--paper); border-color: var(--accent); }
  .quiz-option .katex { font-size: 1em; }

  .quiz-feedback {
    margin-top: 22px;
    padding: 16px 20px;
    border-left: 3px solid;
    font-size: 0.95rem;
    line-height: 1.55;
    display: none;
    background: var(--paper);
  }
  .quiz-feedback.show { display: block; animation: fade .3s ease; }
  .quiz-feedback.correct { border-color: var(--green); }
  .quiz-feedback.correct strong { color: var(--green); }
  .quiz-feedback.wrong { border-color: var(--accent); }
  .quiz-feedback.wrong strong { color: var(--accent); }
  .quiz-feedback .katex { font-size: 1em; }

  .quiz-footer {
    display: flex;
    justify-content: space-between;
    align-items: center;
    margin-top: 28px;
    padding-top: 22px;
    border-top: 1px solid var(--rule);
  }
  .quiz-score {
    font-family: var(--font-mono);
    font-size: 0.85rem;
    color: var(--ink-mute);
  }
  .quiz-score strong {
    font-family: var(--font-serif);
    font-size: 1.4rem;
    color: var(--ink);
  }

  .quiz-finish {
    text-align: center;
    padding: 24px 0;
  }
  .quiz-finish-score {
    font-family: var(--font-serif);
    font-size: 5rem;
    font-weight: 300;
    line-height: 1;
    margin-bottom: 14px;
    color: var(--ink);
  }
  .quiz-finish-score em {
    font-style: italic;
    color: var(--accent);
  }

  /* ============ FOOTER ============ */
  footer {
    border-top: 1px solid var(--rule);
    padding: 48px 0 36px;
    text-align: center;
    margin-top: 60px;
    position: relative;
    z-index: 1;
  }
  footer p {
    color: var(--ink-mute);
    font-size: 0.88rem;
  }
  footer .footer-mark {
    font-family: var(--font-serif);
    font-style: italic;
    font-size: 1.1rem;
    color: var(--ink);
    margin-bottom: 8px;
  }

  /* ============ REVEAL ============ */
  .reveal {
    opacity: 0;
    transform: translateY(20px);
    transition: opacity 0.7s ease, transform 0.7s ease;
  }
  .reveal.visible {
    opacity: 1;
    transform: translateY(0);
  }

  /* Ensure katex renders correctly inside light theme */
  .katex { color: inherit; font-size: 1.05em; }

  ::-webkit-scrollbar { width: 10px; }
  ::-webkit-scrollbar-track { background: var(--paper-soft); }
  ::-webkit-scrollbar-thumb { background: var(--rule); }
  ::-webkit-scrollbar-thumb:hover { background: var(--ink-mute); }

  @media (max-width: 700px) {
    .container, .container-narrow { padding: 0 20px; }
    .panel { padding: 28px 22px; }
    .quiz-card { padding: 28px 22px; }
    .nav-inner { padding: 14px 20px; }
    .nav-links { display: none; }
    .hero { padding: 60px 0 60px; }
    section { padding: 64px 0; }
  }