@import url('https://fonts.googleapis.com/css2?family=DM+Mono:wght@400;500&family=DM+Sans:wght@300;400;500;600;700&display=swap');
  :root {
    --surface:     #162033;
    --card:        #1C2942;
    --danger:      #FF8B8B;
    --warn:        #FFD28C;
    --label:       #A8B6CF;
    --mono:        'DM Mono', monospace;
    --sans:        'DM Sans', sans-serif;
    --radius:      10px;
    --radius-card: 14px;
  }

  body.light {
    --surface:     #FFFFFF;
    --card:        #FFFFFF;
    --danger:      #E63939;
    --warn:        #B45309;
    --label:       #7F8C8D;
    --shadow:      0 2px 16px rgba(120,160,220,0.10);
  }

  /* Light mode overrides for hardcoded dark values */
  body.light .output-preview {
    background: #F0F4FF;
    color: #2E9E7A;
  }
  body.light .assumption-list code,
  body.light .step-row code,
  body.light .hint code,
  body.light .faq-answer code,
  body.light .faq-table td code {
    background: rgba(90,145,232,0.10);
    color: var(--accent);
  }
  body.light .steps-section {
    background: rgba(90,145,232,0.05);
    border-color: rgba(90,145,232,0.15);
  }
  body.light .step-badge {
    background: rgba(90,145,232,0.12);
    border-color: rgba(90,145,232,0.28);
    color: var(--accent);
  }
  body.light .sample-num {
    background: rgba(90,145,232,0.12);
    border-color: rgba(90,145,232,0.25);
    color: var(--accent);
  }
  body.light .ov-type-badge {
    background: rgba(46,158,122,0.12);
    border-color: rgba(46,158,122,0.30);
    color: var(--accent2);
  }
  body.light .dl-json-link {
    background: rgba(46,158,122,0.08);
    border-color: rgba(46,158,122,0.35);
    color: var(--accent2);
  }
  body.light .dl-json-link:hover {
    background: rgba(46,158,122,0.16);
    border-color: var(--accent2);
  }

  /* Theme toggle button */
  .btn-theme {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 3px 10px;
    font-size: 11px;
    font-weight: 600;
    color: var(--muted);
    background: var(--bg);
    cursor: pointer;
    font-family: var(--sans);
    transition: border-color 0.15s, color 0.15s, background 0.15s;
    white-space: nowrap;
  }
  .btn-theme:hover {
    border-color: var(--accent);
    color: var(--text);
  }

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

  body {
    font-family: var(--sans);
    background: var(--bg);
    color: var(--text);
    min-height: 100vh;
    font-size: 14px;
    line-height: 1.5;
    display: flex;
    flex-direction: column;
    transition: background 0.25s, color 0.25s;
  }

  /* ── HEADER ─────────────────────────────────────────────────────── */
  header {
    background: var(--surface);
    border-bottom: 1px solid var(--border);
    padding: 8px 24px 10px;
    display: flex;
    align-items: flex-end;
    justify-content: space-between;
    gap: 12px;
    position: sticky;
    top: 0;
    z-index: 100;
  }
  .header-right {
    display: flex;
    align-items: center;
    gap: 8px;
    flex-shrink: 0;
  }
  .header-left {
    display: flex;
    flex-direction: column;
    align-items: flex-start;
    gap: 8px;
    min-width: 0;
  }
  .header-nav {
    display: flex;
    align-items: center;
    gap: 8px;
    flex-wrap: wrap;
  }
  .header-text { min-width: 0; }
  .header-title {
    font-family: var(--mono);
    font-weight: 600;
    font-size: 15px;
    color: var(--accent);
    letter-spacing: 0.04em;
  }
  .header-subtitle {
    font-size: 12px;
    color: var(--muted);
    margin-top: 2px;
  }
  .back-link {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 3px 8px;
    font-size: 11px;
    font-weight: 600;
    color: var(--text);
    background: var(--bg);
    text-decoration: none;
    transition: border-color 0.15s, color 0.15s;
    white-space: nowrap;
  }
  .back-link:hover { border-color: var(--accent); color: var(--accent); }

  .dl-json-link {
    display: inline-flex;
    align-items: center;
    gap: 4px;
    border: 1px solid rgba(143,205,189,0.35);
    border-radius: var(--radius);
    padding: 2px 8px;
    font-size: 11px;
    font-weight: 600;
    color: var(--accent2);
    background: rgba(143,205,189,0.08);
    text-decoration: none;
    transition: border-color 0.15s, background 0.15s;
    white-space: nowrap;
    flex-shrink: 0;
  }
  .dl-json-link:hover { border-color: var(--accent2); background: rgba(143,205,189,0.16); }

  /* ── MAIN CONTENT ────────────────────────────────────────────────── */
  main {
    flex: 1;
    max-width: 860px;
    width: 100%;
    margin: 0 auto;
    padding: 32px 24px 60px;
  }

  /* ── PAGE HERO ───────────────────────────────────────────────────── */
  .page-hero {
    margin-bottom: 32px;
  }
  .page-hero h1 {
    font-family: var(--mono);
    font-size: 22px;
    font-weight: 600;
    color: var(--accent);
    letter-spacing: 0.03em;
    margin-bottom: 8px;
  }
  .page-hero p {
    font-size: 13.5px;
    color: var(--muted);
    line-height: 1.65;
    max-width: 640px;
  }

  /* ── SECTION CARDS ───────────────────────────────────────────────── */
  .section-card {
    background: var(--card);
    border: 1px solid var(--border);
    border-radius: var(--radius-card);
    margin-bottom: 20px;
    box-shadow: var(--shadow);
    overflow: visible;
  }
  .section-header {
    display: flex;
    align-items: center;
    gap: 10px;
    padding: 12px 16px;
    background: var(--surface);
    border-bottom: 1px solid var(--border);
    border-radius: var(--radius-card) var(--radius-card) 0 0;
    cursor: pointer;
    user-select: none;
  }
  .section-header.collapsed {
    border-bottom: none;
    border-radius: var(--radius-card);
  }
  .section-header h3 {
    font-size: 14px;
    font-weight: 600;
    color: var(--text);
    flex: 1;
    margin: 0;
    text-transform: none;
    letter-spacing: normal;
  }
  .toggle-icon {
    color: var(--muted);
    font-size: 12px;
    transition: transform 0.2s;
    flex-shrink: 0;
  }
  .section-header.collapsed .toggle-icon { transform: rotate(-90deg); }
  .section-body { padding: 20px; }
  .section-body.hidden { display: none; }

  /* Sample number badge */
  .sample-num {
    font-family: var(--mono);
    font-size: 11px;
    font-weight: 600;
    color: var(--accent);
    background: rgba(141,187,255,0.12);
    border: 1px solid rgba(141,187,255,0.25);
    border-radius: 6px;
    padding: 2px 7px;
    flex-shrink: 0;
  }

  /* ── SUB-SECTIONS ────────────────────────────────────────────────── */
  .sub-section {
    margin-bottom: 24px;
  }
  .sub-section:last-child { margin-bottom: 0; }
  .sub-section-title {
    display: flex;
    align-items: center;
    gap: 8px;
    font-size: 11px;
    font-weight: 700;
    color: var(--muted);
    text-transform: uppercase;
    letter-spacing: 0.08em;
    margin-bottom: 12px;
    padding-bottom: 6px;
    border-bottom: 1px solid var(--border);
  }
  .step-badge {
    display: inline-flex;
    align-items: center;
    justify-content: center;
    width: 20px;
    height: 20px;
    border-radius: 50%;
    background: rgba(141,187,255,0.12);
    border: 1px solid rgba(141,187,255,0.28);
    font-size: 12px;
    color: var(--accent);
    flex-shrink: 0;
  }

  /* ── OBJECTIVE / ASSUMPTION TEXT ─────────────────────────────────── */
  .objective-text {
    font-size: 13.5px;
    color: var(--text);
    line-height: 1.7;
  }
  .assumption-list {
    list-style: none;
    padding: 0;
  }
  .assumption-list li {
    font-size: 13px;
    color: var(--text);
    line-height: 1.65;
    padding: 3px 0 3px 18px;
    position: relative;
  }
  .assumption-list li::before {
    content: '-';
    position: absolute;
    left: 0;
    color: var(--muted);
  }
  .assumption-list code {
    font-family: var(--mono);
    font-size: 12px;
    color: var(--accent);
    background: rgba(141,187,255,0.1);
    border-radius: 4px;
    padding: 1px 5px;
  }

  /* ── READ-ONLY FIELDS ────────────────────────────────────────────── */
  .readonly-fields {
    pointer-events: none;
  }
  .readonly-group-title {
    font-size: 11px;
    font-weight: 700;
    color: var(--muted);
    text-transform: uppercase;
    letter-spacing: 0.07em;
    margin: 16px 0 8px;
  }
  .readonly-group-title:first-child { margin-top: 0; }

  .form-row {
    display: grid;
    grid-template-columns: 140px 1fr;
    align-items: start;
    gap: 8px;
    margin-bottom: 8px;
  }
  label {
    font-size: 12px;
    color: var(--label);
    padding-top: 7px;
    font-weight: 500;
  }
  input[type="text"],
  input[type="number"],
  select {
    background: var(--bg);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    color: var(--text);
    font-family: var(--sans);
    font-size: 13px;
    padding: 6px 10px;
    width: 100%;
    opacity: 0.85;
    cursor: not-allowed;
    outline: none;
  }
  input[type="checkbox"] {
    width: 15px;
    height: 15px;
    accent-color: var(--accent);
  }
  select option { background: var(--card); }

  /* ── DYNAMIC TABLES ──────────────────────────────────────────────── */
  .dyn-table-wrapper { overflow-x: auto; }
  .dyn-table {
    width: 100%;
    border-collapse: collapse;
    font-size: 12px;
    min-width: 500px;
  }
  .dyn-table th {
    background: var(--surface);
    color: var(--label);
    font-weight: 600;
    font-size: 11px;
    text-transform: uppercase;
    letter-spacing: 0.06em;
    padding: 6px 8px;
    text-align: center;
    border-bottom: 1px solid var(--border);
    white-space: nowrap;
  }
  .dyn-table td {
    padding: 5px 4px;
    vertical-align: middle;
    border-bottom: 1px solid var(--border);
    text-align: center;
  }
  .dyn-table tbody tr:last-child td { border-bottom: none; }
  .dyn-table td input,
  .dyn-table td select {
    padding: 5px 6px;
    font-size: 12px;
    width: 100%;
    min-width: 50px;
    text-align: center;
  }

  /* ── OUTPUT VAR CARD ─────────────────────────────────────────────── */
  .output-var-item {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius-card);
    overflow: hidden;
    margin-bottom: 8px;
  }
  .output-var-header {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 8px 12px;
    background: var(--bg);
    border-bottom: 1px solid var(--border);
  }
  .ov-type-badge {
    font-family: var(--mono);
    font-size: 11px;
    font-weight: 600;
    color: var(--accent2);
    background: rgba(143,205,189,0.12);
    border: 1px solid rgba(143,205,189,0.3);
    border-radius: 5px;
    padding: 2px 7px;
    flex-shrink: 0;
  }
  .output-var-body { padding: 10px 12px; }

  /* ── STEPS ───────────────────────────────────────────────────────── */
  .steps-section {
    background: rgba(141,187,255,0.04);
    border: 1px solid rgba(141,187,255,0.12);
    border-radius: var(--radius);
    padding: 14px 16px !important;
  }
  .steps-title {
    font-size: 11px;
    font-weight: 700;
    color: var(--muted);
    text-transform: uppercase;
    letter-spacing: 0.08em;
    margin-bottom: 12px;
  }
  .step-row {
    display: flex;
    align-items: flex-start;
    gap: 10px;
    padding: 5px 0;
    font-size: 13px;
    color: var(--text);
  }
  .step-row .step-badge { margin-top: 1px; flex-shrink: 0; }
  .step-row strong { color: var(--accent); }
  .step-row code {
    font-family: var(--mono);
    font-size: 12px;
    color: var(--accent);
    background: rgba(141,187,255,0.1);
    border-radius: 4px;
    padding: 1px 5px;
  }
  .step-note {
    font-size: 12px;
    color: var(--muted);
    margin-top: 4px;
    line-height: 1.5;
  }

  /* ── EXPECTED OUTPUT ─────────────────────────────────────────────── */
  .output-preview {
    background: #0B1120;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 14px 16px;
    font-family: var(--mono);
    font-size: 12px;
    color: var(--accent2);
    line-height: 1.8;
    overflow-x: auto;
    margin: 10px 0;
    white-space: pre;
  }
  .hint {
    font-size: 12.5px;
    color: var(--muted);
    line-height: 1.6;
  }
  .hint code {
    font-family: var(--mono);
    font-size: 11.5px;
    color: var(--accent);
    background: rgba(141,187,255,0.1);
    border-radius: 4px;
    padding: 1px 5px;
  }

  /* ── GRAPH FIGURES ───────────────────────────────────────────────── */
  .graph-figure-row {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 14px;
    margin: 14px 0;
  }
  .graph-figure {
    margin: 0;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    background: #fff;
    overflow: hidden;
  }
  .graph-figure img {
    display: block;
    width: 100%;
    height: auto;
  }
  .graph-figure figcaption {
    font-size: 12px;
    color: var(--muted);
    line-height: 1.5;
    padding: 8px 12px 10px;
    border-top: 1px solid var(--border);
  }
  .graph-figure figcaption code {
    font-family: var(--mono);
    font-size: 11px;
    color: var(--accent);
    background: rgba(141,187,255,0.1);
    border-radius: 4px;
    padding: 1px 5px;
  }
  @media (max-width: 640px) {
    .graph-figure-row { grid-template-columns: 1fr; }
  }

  /* ── FAQ ─────────────────────────────────────────────────────────── */
  .faq-list { margin-top: 4px; }
  .faq-item {
    border-bottom: 1px solid var(--border);
  }
  .faq-item:last-child { border-bottom: none; }
  .faq-question {
    display: flex;
    align-items: center;
    gap: 8px;
    padding: 13px 4px;
    cursor: pointer;
    user-select: none;
    font-size: 13.5px;
    font-weight: 600;
    color: var(--text);
    transition: color 0.15s;
  }
  .faq-question:hover { color: var(--accent); }
  .faq-toggle {
    color: var(--muted);
    font-size: 12px;
    margin-left: auto;
    transition: transform 0.2s;
    flex-shrink: 0;
  }
  .faq-question.open .faq-toggle { transform: rotate(180deg); }
  .faq-answer {
    display: none;
    padding: 0 4px 16px;
  }
  .faq-answer.open { display: block; }
  .faq-answer p {
    font-size: 13px;
    color: var(--muted);
    line-height: 1.65;
    margin-bottom: 8px;
  }
  .faq-answer p:last-child { margin-bottom: 0; }
  .faq-answer code {
    font-family: var(--mono);
    font-size: 12px;
    color: var(--accent);
    background: rgba(141,187,255,0.1);
    border-radius: 4px;
    padding: 1px 5px;
  }
  .faq-answer strong { color: var(--text); font-weight: 600; }

  .faq-table-wrapper {
    overflow-x: auto;
    margin-top: 10px;
  }
  .faq-table {
    width: 100%;
    border-collapse: collapse;
    font-size: 12.5px;
    border: 1px solid var(--border);
    border-radius: var(--radius);
    overflow: hidden;
  }
  .faq-table th {
    background: var(--surface);
    color: var(--label);
    font-weight: 600;
    font-size: 11px;
    text-transform: uppercase;
    letter-spacing: 0.05em;
    padding: 6px 12px;
    text-align: left;
    border-bottom: 1px solid var(--border);
  }
  .faq-table td {
    padding: 8px 12px;
    border-bottom: 1px solid var(--border);
    color: var(--muted);
    vertical-align: top;
    line-height: 1.5;
  }
  .faq-table tbody tr:last-child td { border-bottom: none; }
  .faq-table td:first-child {
    color: var(--text);
    font-weight: 500;
    white-space: nowrap;
  }
  .faq-table td code {
    font-family: var(--mono);
    font-size: 11.5px;
    color: var(--accent);
    background: rgba(141,187,255,0.1);
    border-radius: 4px;
    padding: 1px 5px;
  }

  /* ── FOOTER ──────────────────────────────────────────────────────── */
  .app-footer {
    border-top: 1px solid var(--border);
    padding: 10px 20px 12px;
    background: var(--bg);
    flex-shrink: 0;
  }
  .app-footer p {
    font-size: 11px;
    color: var(--muted);
    line-height: 1.6;
    margin-bottom: 5px;
  }
  .app-footer a { color: var(--accent); text-decoration: none; }
  .app-footer a:hover { text-decoration: underline; }

  /* ── SCROLLBAR ───────────────────────────────────────────────────── */
  ::-webkit-scrollbar { width: 6px; height: 6px; }
  ::-webkit-scrollbar-track { background: transparent; }
  ::-webkit-scrollbar-thumb { background: var(--border); border-radius: 3px; }
  ::-webkit-scrollbar-thumb:hover { background: var(--accent); }

  /* ── MOBILE ──────────────────────────────────────────────────────── */
  @media (max-width: 768px) {
    header { padding: 8px 12px; align-items: flex-start; }
    .header-left { gap: 6px; }
    .header-nav { gap: 6px; }
    main { padding: 20px 14px 40px; }
    .page-hero h1 { font-size: 18px; }
    .form-row { grid-template-columns: 1fr; }
    .form-row label { padding-top: 0; padding-bottom: 2px; }
    .dyn-table-wrapper { overflow-x: auto; }
    .dyn-table { min-width: 420px; }
    .faq-table { font-size: 12px; }
  }

  /* ── BUILT-IN MODELS GRID ────────────────────────────────────────── */
  .model-grid {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 14px;
    margin-top: 14px;
  }
  .model-item {
    background: var(--surface);
    border: 1px solid var(--border);
    border-radius: var(--radius);
    padding: 12px 14px;
  }
  .model-head {
    display: flex;
    align-items: center;
    gap: 8px;
    margin-bottom: 8px;
    font-size: 13.5px;
    color: var(--text);
  }
  .model-badge {
    font-family: var(--mono);
    font-size: 11px;
    font-weight: 600;
    color: var(--accent2);
    background: rgba(126,231,196,0.10);
    border: 1px solid rgba(126,231,196,0.28);
    border-radius: 6px;
    padding: 2px 7px;
    flex-shrink: 0;
  }
  .model-item p {
    font-size: 12.5px;
    color: var(--muted);
    line-height: 1.6;
    margin: 0;
  }
  @media (max-width: 720px) {
    .model-grid { grid-template-columns: 1fr; }
  }