:root{
    --bg:#F6F7F9;
    --surface:#FFFFFF;
    --surface-2:#EEF1F6;
    --surface-3:#E5E9F0;
    --ink:#13212F;
    --ink-soft:#4C5A6E;
    --ink-faint:#7C889A;
    --border:#DBE1EA;
    --border-strong:#C2CADA;
    --navy:#0A2342;
    --navy-ink:#0A2342;
    --navy-soft:#3A5680;
    --orange:#C96A16;
    --orange-fill:#F28C28;
    --orange-bg:#FCEEDD;
    --teal:#00707F;
    --teal-bg:#E1F1F3;
    --warn:#9A2B2B;
    --warn-bg:#FBEAEA;
    --warn-border:#E7B9B9;
    --ok:#1F7A4D;
    --ok-bg:#E7F5EC;
    --shadow: 0 1px 2px rgba(10,35,66,0.06), 0 8px 24px -12px rgba(10,35,66,0.18);
    --focus: 0 0 0 3px rgba(0,123,143,0.25);
  }
  @media (prefers-color-scheme: dark){
    :root:not([data-theme="light"]){
      --bg:#0C1420;
      --surface:#121C2B;
      --surface-2:#182437;
      --surface-3:#20304A;
      --ink:#E8ECF2;
      --ink-soft:#A9B6C8;
      --ink-faint:#7C889A;
      --border:#293651;
      --border-strong:#3A4A6B;
      --navy:#7FA8D9;
      --navy-ink:#0D1420;
      --navy-soft:#9DBCE3;
      --orange:#F3A867;
      --orange-fill:#F0A15A;
      --orange-bg:#3A2A18;
      --teal:#5BC4D3;
      --teal-bg:#123338;
      --warn:#E79999;
      --warn-bg:#3A1717;
      --warn-border:#5C2A2A;
      --ok:#6FCB98;
      --ok-bg:#123122;
      --shadow: 0 1px 2px rgba(0,0,0,0.4), 0 8px 24px -12px rgba(0,0,0,0.6);
      --focus: 0 0 0 3px rgba(91,196,211,0.3);
    }
  }
  :root[data-theme="dark"]{
    --bg:#0C1420;
    --surface:#121C2B;
    --surface-2:#182437;
    --surface-3:#20304A;
    --ink:#E8ECF2;
    --ink-soft:#A9B6C8;
    --ink-faint:#7C889A;
    --border:#293651;
    --border-strong:#3A4A6B;
    --navy:#7FA8D9;
    --navy-ink:#0D1420;
    --navy-soft:#9DBCE3;
    --orange:#F3A867;
    --orange-fill:#F0A15A;
    --orange-bg:#3A2A18;
    --teal:#5BC4D3;
    --teal-bg:#123338;
    --warn:#E79999;
    --warn-bg:#3A1717;
    --warn-border:#5C2A2A;
    --ok:#6FCB98;
    --ok-bg:#123122;
    --shadow: 0 1px 2px rgba(0,0,0,0.4), 0 8px 24px -12px rgba(0,0,0,0.6);
    --focus: 0 0 0 3px rgba(91,196,211,0.3);
  }

  *{ box-sizing:border-box; }
  html{ background:var(--bg); }
  body{
    margin:0;
    background:var(--bg);
    color:var(--ink);
    font-family:"Public Sans", ui-sans-serif, system-ui, -apple-system, "Segoe UI", Arial, sans-serif;
    font-size:16px;
    line-height:1.6;
    -webkit-font-smoothing:antialiased;
  }
  h1,h2,h3{
    font-family:"Source Serif 4", Georgia, "Times New Roman", serif;
    color:var(--navy);
    text-wrap:balance;
    margin:0;
  }
  .mono{ font-family:"IBM Plex Mono", ui-monospace, SFMono-Regular, Menlo, monospace; }
  a{ color:var(--teal); }
  button{ font-family:inherit; }
  ::selection{ background:var(--orange-bg); }

  /* ---------- Skip link / focus ---------- */
  textarea:focus-visible, input:focus-visible, button:focus-visible, [tabindex]:focus-visible{
    outline:none; box-shadow:var(--focus); border-color:var(--teal);
  }

  /* ---------- Sticky toolbar ---------- */
  .toolbar{
    position:sticky; top:0; z-index:40;
    display:flex; align-items:center; gap:18px;
    padding:10px 20px;
    background:color-mix(in srgb, var(--surface) 88%, transparent);
    backdrop-filter:blur(8px);
    border-bottom:1px solid var(--border);
    flex-wrap:wrap;
  }
  .toolbar .tb-group{ display:flex; align-items:center; gap:8px; }
  .tb-label{ font-size:11px; letter-spacing:0.06em; text-transform:uppercase; color:var(--ink-faint); }
  .tb-time{ font-size:20px; font-weight:600; font-variant-numeric:tabular-nums; color:var(--navy); min-width:64px; }
  .tb-phase{
    font-size:11px; letter-spacing:0.06em; text-transform:uppercase; font-weight:600;
    padding:3px 9px; border-radius:100px; background:var(--surface-3); color:var(--ink-soft);
  }
  .tb-phase[data-phase="answer"]{ background:var(--teal-bg); color:var(--teal); }
  .tb-phase[data-phase="review"]{ background:var(--orange-bg); color:var(--orange); }
  .tb-phase[data-phase="done"]{ background:var(--warn-bg); color:var(--warn); }
  .tb-btn{
    border:1px solid var(--border-strong); background:var(--surface); color:var(--ink);
    padding:7px 14px; border-radius:8px; font-size:13px; font-weight:600; cursor:pointer;
    transition:background .15s, transform .1s;
  }
  .tb-btn:hover{ background:var(--surface-2); }
  .tb-btn:active{ transform:translateY(1px); }
  .tb-btn.primary{ background:var(--navy); color:var(--surface); border-color:var(--navy); }
  .tb-btn.primary:hover{ filter:brightness(1.08); }
  .tb-btn:disabled{ opacity:.45; cursor:not-allowed; }
  .tb-spacer{ flex:1 1 auto; }
  .tb-words{ font-size:13px; color:var(--ink-soft); }
  .tb-words strong{ color:var(--ink); font-variant-numeric:tabular-nums; }
  .tb-words.over{ color:var(--warn); }
  .tb-words.ok{ color:var(--ok); }

  /* ---------- Page shell ---------- */
  .sheet{ max-width:840px; margin:0 auto; padding:0 20px 80px; }

  /* ---------- Header banner ---------- */
  .banner{
    background:var(--navy-ink, var(--navy));
    background-color:#0A2342;
    color:#EAF0F8;
    margin:0 -20px 28px;
    padding:34px 26px 28px;
  }
  @media (prefers-color-scheme: dark){ :root:not([data-theme="light"]) .banner{ background-color:#080F19; } }
  :root[data-theme="dark"] .banner{ background-color:#080F19; }
  .banner-top{ display:flex; align-items:flex-start; justify-content:space-between; gap:24px; flex-wrap:wrap; }
  .banner-logo{ height:34px; width:auto; display:block; }
  .banner-kicker{ font-size:12px; letter-spacing:0.08em; text-transform:uppercase; color:#9FB6D6; font-weight:600; }
  .banner-programme{ font-size:13.5px; color:#C6D4E6; font-style:italic; margin-top:3px; }
  .banner h1{ color:#FFFFFF; font-size:30px; font-weight:600; margin-top:16px; }
  .banner-tag{
    display:inline-block; margin-top:10px; font-size:12px; letter-spacing:0.07em; text-transform:uppercase;
    color:#F6C88C; font-weight:600;
  }
  .banner-meta{ display:flex; gap:8px; flex-wrap:wrap; }
  .chip{
    font-size:11.5px; letter-spacing:0.03em; font-weight:600;
    padding:5px 11px; border-radius:100px; border:1px solid rgba(255,255,255,0.22);
    color:#EAF0F8; background:rgba(255,255,255,0.06); white-space:nowrap;
  }
  .chip.accent{ background:rgba(242,140,40,0.18); border-color:rgba(242,140,40,0.45); color:#F6C88C; }
  .banner-rule{ margin-top:20px; padding-top:14px; border-top:1px solid rgba(255,255,255,0.16);
    font-size:11.5px; letter-spacing:0.08em; text-transform:uppercase; color:#B9C8DD; font-weight:600; }

  /* ---------- Section primitives ---------- */
  .section{ margin:0 0 34px; }
  .section-head{
    display:flex; align-items:baseline; gap:10px; margin-bottom:14px;
    border-bottom:2px solid var(--navy); padding-bottom:8px;
  }
  .section-head h2{ font-size:21px; }
  .section-num{ font-family:"IBM Plex Mono"; font-size:12px; color:var(--ink-faint); }
  p{ margin:0 0 12px; color:var(--ink); }
  p:last-child{ margin-bottom:0; }
  .lede{ color:var(--ink-soft); }

  ul.obj{ margin:0; padding:0; list-style:none; display:grid; gap:10px; }
  ul.obj li{
    position:relative; padding-left:24px; color:var(--ink);
  }
  ul.obj li::before{
    content:""; position:absolute; left:0; top:9px; width:8px; height:8px; border-radius:2px;
    background:var(--teal); transform:rotate(45deg);
  }

  /* ---------- Student details ---------- */
  .id-card{
    background:var(--surface); border:1px solid var(--border); border-radius:12px;
    padding:20px 22px; box-shadow:var(--shadow); margin-top:-46px; position:relative; z-index:2;
    display:grid; grid-template-columns:repeat(auto-fit, minmax(190px,1fr)); gap:16px 20px;
  }
  .field label{ display:block; font-size:11px; letter-spacing:0.06em; text-transform:uppercase; color:var(--ink-faint); font-weight:600; margin-bottom:6px; }
  .field input{
    width:100%; font-size:15px; padding:9px 10px; border-radius:7px; border:1px solid var(--border-strong);
    background:var(--bg); color:var(--ink); font-family:inherit;
  }
  .field input:focus{ background:var(--surface); }
  .field.locked .lock-value{
    display:inline-flex; align-items:center; gap:6px; font-weight:600; color:var(--navy);
    font-size:15px; padding:9px 2px;
  }
  .field.locked svg{ width:13px; height:13px; opacity:.6; }

  /* ---------- Equation callout ---------- */
  .equation{
    background:var(--teal-bg); border:1px solid color-mix(in srgb, var(--teal) 35%, var(--border));
    border-radius:12px; padding:18px 20px; display:flex; gap:16px; align-items:flex-start;
  }
  .equation .eq-icon{ flex:0 0 auto; margin-top:2px; }
  .equation .eq-icon svg{ width:22px; height:22px; color:var(--teal); }
  .equation strong{ color:var(--navy); }
  .eq-terms{ display:flex; flex-wrap:wrap; gap:6px 8px; margin-top:10px; }
  .eq-term{
    font-family:"IBM Plex Mono"; font-size:11.5px; font-weight:600; letter-spacing:0.02em;
    padding:3px 9px; border-radius:6px; background:var(--surface); border:1px solid var(--border);
    color:var(--navy);
  }

  /* ---------- Case narrative ---------- */
  .case-title{ font-size:19px; margin-bottom:4px; color:var(--orange); font-style:italic; }
  .case-body p{ font-size:15.5px; }

  .exhibit{
    background:var(--surface); border:1px solid var(--border); border-radius:12px; margin:18px 0;
    overflow:hidden; box-shadow:var(--shadow);
  }
  .exhibit-head{
    display:flex; align-items:center; gap:8px; padding:11px 16px; background:var(--surface-2);
    border-bottom:1px solid var(--border);
  }
  .exhibit-head .ex-tag{
    font-family:"IBM Plex Mono"; font-size:10.5px; font-weight:600; letter-spacing:0.05em;
    color:var(--teal); background:var(--teal-bg); padding:2px 8px; border-radius:5px; text-transform:uppercase;
  }
  .exhibit-head h3{ font-size:14.5px; font-family:"Public Sans"; font-weight:700; color:var(--ink); }
  .exhibit-body{ padding:14px 18px 16px; display:grid; gap:10px; }
  .msg{ display:flex; flex-direction:column; gap:2px; max-width:88%; }
  .msg .who{ font-size:11.5px; font-weight:700; color:var(--navy); }
  .msg .meta{ font-weight:500; color:var(--ink-faint); font-size:10.5px; margin-left:6px; }
  .msg .bubble{
    background:var(--surface-2); border:1px solid var(--border); border-radius:10px;
    padding:9px 12px; font-size:14.5px; font-style:italic; color:var(--ink);
  }
  .msg.priya .bubble{ background:var(--teal-bg); border-color:color-mix(in srgb, var(--teal) 30%, var(--border)); }
  .msg.rohan{ align-self:flex-end; align-items:flex-end; }
  .msg.rohan .bubble{ background:var(--orange-bg); border-color:color-mix(in srgb, var(--orange) 30%, var(--border)); }
  .msg.note .bubble{ font-style:normal; color:var(--ink-faint); background:transparent; border-style:dashed; }

  /* ---------- Assignment parts ---------- */
  .part{
    border-radius:12px; border:1px solid var(--border); background:var(--surface);
    box-shadow:var(--shadow); margin-bottom:20px; overflow:hidden;
    border-left:5px solid var(--part-color, var(--navy));
  }
  .part[data-part="A"]{ --part-color:var(--teal); }
  .part[data-part="B"]{ --part-color:var(--orange-fill); }
  .part[data-part="C"]{ --part-color:var(--navy); }
  .part-head{ padding:15px 20px 12px; border-bottom:1px solid var(--border); }
  .part-head h3{ font-size:17px; }
  .part-head .part-eyebrow{ font-family:"IBM Plex Mono"; font-size:11px; letter-spacing:0.05em; color:var(--part-color); font-weight:600; text-transform:uppercase; }
  .part-intro{ padding:12px 20px 0; color:var(--ink-soft); font-size:14.5px; }
  .qa{ padding:16px 20px; border-top:1px dashed var(--border); }
  .qa:first-of-type{ border-top:none; }
  .q-row{ display:flex; gap:10px; margin-bottom:10px; }
  .q-id{
    flex:0 0 auto; font-family:"IBM Plex Mono"; font-weight:700; font-size:13px;
    color:#fff; background:var(--part-color); width:30px; height:30px; border-radius:8px;
    display:flex; align-items:center; justify-content:center;
  }
  .q-text{ padding-top:4px; font-size:15px; }
  .q-hint{ font-size:12px; color:var(--ink-faint); margin:2px 0 8px 40px; }
  textarea.ans{
    width:100%; box-sizing:border-box; resize:vertical; min-height:64px;
    border:1px solid var(--border-strong); border-radius:9px; background:var(--bg);
    color:var(--ink); font:inherit; font-size:14.5px; padding:11px 13px; line-height:1.55;
    font-family:"Public Sans";
  }
  textarea.ans:focus{ background:var(--surface); }
  .word-note{ display:flex; justify-content:flex-end; margin-top:5px; }
  .word-note span{ font-size:11.5px; color:var(--ink-faint); font-variant-numeric:tabular-nums; }

  /* ---------- Submission guidelines ---------- */
  .guide-list{ margin:0; padding:0; list-style:none; display:grid; gap:9px; }
  .guide-list li{ position:relative; padding-left:22px; font-size:14.5px; }
  .guide-list li::before{ content:"→"; position:absolute; left:0; color:var(--orange); font-weight:700; }

  /* ---------- Integrity / AI policy ---------- */
  .integrity{
    background:var(--warn-bg); border:1px solid var(--warn-border); border-radius:12px; padding:20px 22px;
  }
  .integrity-head{ display:flex; align-items:center; gap:10px; margin-bottom:10px; }
  .integrity-head svg{ width:20px; height:20px; color:var(--warn); flex:0 0 auto; }
  .integrity-head h2{ color:var(--warn); font-size:18px; }
  .integrity p{ font-size:14.5px; color:var(--ink); }
  .integrity strong{ color:var(--warn); }
  .decl{
    margin-top:14px; padding-top:14px; border-top:1px solid var(--warn-border);
    display:flex; align-items:flex-start; gap:11px;
  }
  .decl input[type="checkbox"]{ width:19px; height:19px; margin-top:2px; accent-color:var(--warn); flex:0 0 auto; }
  .decl label{ font-size:14px; color:var(--ink); }
  .decl label b{ display:block; font-size:12.5px; color:var(--ink-faint); font-weight:500; margin-top:3px; }

  /* ---------- Footer ---------- */
  .foot{
    margin-top:36px; padding-top:18px; border-top:1px solid var(--border);
    display:flex; justify-content:space-between; gap:12px; flex-wrap:wrap;
    font-size:12px; color:var(--ink-faint);
  }

  /* ---------- Print ---------- */
  @media print{
    .toolbar{ display:none !important; }
    body{ background:#fff; }
    .banner{ background:#0A2342 !important; -webkit-print-color-adjust:exact; print-color-adjust:exact; }
    .id-card, .exhibit, .part, .equation, .integrity{ box-shadow:none !important; break-inside:avoid; }
    .part{ break-inside:avoid-page; }
    textarea.ans{ border:1px solid #99a3b3; background:#fff !important; overflow:hidden; }
    a{ color:inherit; text-decoration:none; }
    .sheet{ max-width:100%; }
  }

  @media (max-width:600px){
    .banner h1{ font-size:24px; }
    .toolbar{ gap:10px; padding:8px 12px; }
    .tb-words{ display:none; }
  }