@import url('https://fonts.googleapis.com/css2?family=Big+Shoulders+Display:wght@600;700;800&family=IBM+Plex+Sans:wght@400;500;600;700&family=IBM+Plex+Mono:wght@400;500;600&display=swap');

  :root{
    --bg:#f2f3f6;
    --surface:#ffffff;
    --surface-2:#e8eaef;
    --border:#d7dbe3;
    --text:#161a22;
    --text-dim:#5a6072;
    --text-faint:#8890a3;
    --ember:#c9480a;
    --ember-soft:#fbe6d8;
    --cyan:#00697f;
    --cyan-soft:#dcf1f5;
    --success:#1f8a4c;
    --shadow: 0 1px 2px rgba(20,24,34,.06), 0 8px 24px rgba(20,24,34,.06);
  }
  @media (prefers-color-scheme: dark){
    :root:not([data-theme="light"]){
      --bg:#0d0f13;
      --surface:#161920;
      --surface-2:#1d212b;
      --border:#2a2f3a;
      --text:#eef0f4;
      --text-dim:#a3aabb;
      --text-faint:#6b7180;
      --ember:#ff8c42;
      --ember-soft:#2c1a10;
      --cyan:#3ecbe8;
      --cyan-soft:#0f2830;
      --success:#4ade80;
      --shadow: 0 1px 2px rgba(0,0,0,.4), 0 8px 28px rgba(0,0,0,.45);
    }
  }
  :root[data-theme="dark"]{
    --bg:#0d0f13;
    --surface:#161920;
    --surface-2:#1d212b;
    --border:#2a2f3a;
    --text:#eef0f4;
    --text-dim:#a3aabb;
    --text-faint:#6b7180;
    --ember:#ff8c42;
    --ember-soft:#2c1a10;
    --cyan:#3ecbe8;
    --cyan-soft:#0f2830;
    --success:#4ade80;
    --shadow: 0 1px 2px rgba(0,0,0,.4), 0 8px 28px rgba(0,0,0,.45);
  }

  *{box-sizing:border-box;}
  html{scroll-behavior:smooth;}
  body{
    margin:0;
    background:var(--bg);
    color:var(--text);
    font-family:'IBM Plex Sans', system-ui, sans-serif;
    font-size:16.5px;
    line-height:1.65;
  }
  ::selection{ background: var(--cyan); color:#fff; }
  a{ color: var(--cyan); }
  h1,h2,h3,h4{ font-family:'Big Shoulders Display', system-ui, sans-serif; text-wrap: balance; letter-spacing:.2px; margin:0; }
  code, pre, .mono{ font-family:'IBM Plex Mono', ui-monospace, monospace; }
  p{ margin:0 0 1em; max-width: 66ch; color: var(--text); }
  ul,ol{ margin:0 0 1em; padding-left:1.3em; max-width:64ch; }
  li{ margin-bottom:.4em; }

  /* ---------- shell layout ---------- */
  .shell{ display:flex; min-height:100vh; }

  /* progress rail */
  .progress-rail{ position:fixed; top:0; left:0; right:0; height:3px; background:var(--surface-2); z-index:60; }
  .progress-fill{ height:100%; width:0%; background: linear-gradient(90deg, var(--cyan), var(--ember)); transition: width .25s ease; }

  /* sidebar */
  .sidebar{
    width:272px; flex:none; position:sticky; top:0; height:100vh; overflow-y:auto;
    background:var(--surface); border-right:1px solid var(--border); padding:20px 0 40px;
    z-index:40;
  }
  .brand{ padding:6px 20px 18px; border-bottom:1px solid var(--border); margin-bottom:8px; }
  .brand .mark{ display:flex; align-items:center; gap:9px; }
  .brand .bolt{ width:20px; height:20px; flex:none; }
  .brand .name{ font-family:'Big Shoulders Display'; font-weight:800; font-size:20px; letter-spacing:.3px; }
  .brand .tag{ color:var(--text-faint); font-size:12px; margin-top:4px; letter-spacing:.4px; text-transform:uppercase; }

  .rail-stat{ padding:12px 20px 16px; border-bottom:1px solid var(--border); margin-bottom:6px;}
  .rail-stat .num{ font-family:'Big Shoulders Display'; font-weight:700; font-size:26px; color:var(--ember); font-variant-numeric: tabular-nums;}
  .rail-stat .lbl{ color:var(--text-faint); font-size:11.5px; text-transform:uppercase; letter-spacing:.5px; }
  .rail-bar{ height:5px; border-radius:3px; background:var(--surface-2); margin-top:8px; overflow:hidden; }
  .rail-bar > i{ display:block; height:100%; width:0%; background: linear-gradient(90deg, var(--cyan), var(--ember)); }

  nav.toc{ padding:6px 10px; }
  nav.toc a{
    display:flex; gap:10px; align-items:baseline; padding:7px 10px; border-radius:8px;
    text-decoration:none; color:var(--text-dim); font-size:13.6px; font-weight:500;
  }
  nav.toc a .n{ font-family:'IBM Plex Mono'; font-size:11px; color:var(--text-faint); flex:none; width:20px; }
  nav.toc a:hover{ background:var(--surface-2); color:var(--text); }
  nav.toc a.active{ background:var(--cyan-soft); color:var(--cyan); }
  nav.toc a.active .n{ color:var(--cyan); }
  nav.toc a.seen .n::after{ content:''; }
  nav.toc a .check{ margin-left:auto; width:13px; height:13px; flex:none; opacity:0; color:var(--success); }
  nav.toc a.seen .check{ opacity:1; }
  .toc-group-label{ font-size:10.5px; text-transform:uppercase; letter-spacing:.7px; color:var(--text-faint); padding:14px 12px 4px; }

  .sidebar-toggle{ display:none; }

  /* main column */
  main{ flex:1; min-width:0; padding: 56px 40px 120px; }
  .content{ max-width:760px; margin:0 auto; }

  .hero{ margin-bottom:64px; }
  .hero .eyebrow{ display:inline-flex; align-items:center; gap:7px; font-size:12px; letter-spacing:.6px; text-transform:uppercase; color:var(--ember); font-weight:600; margin-bottom:14px;}
  .hero h1{ font-size:56px; font-weight:800; line-height:1.0; margin-bottom:16px; }
  .hero .lede{ font-size:18.5px; color:var(--text-dim); max-width:56ch; }
  .hero .meta-row{ display:flex; gap:22px; flex-wrap:wrap; margin-top:26px; }
  .hero .meta{ font-size:13px; color:var(--text-faint); display:flex; align-items:center; gap:7px; }
  .hero .meta b{ color:var(--text); font-weight:600; }

  section.module{ padding-top:56px; margin-top:8px; border-top:1px solid var(--border); scroll-margin-top: 24px; }
  section.module:first-of-type{ border-top:none; }
  .module-kicker{ display:flex; align-items:center; gap:12px; margin-bottom:10px; }
  .module-kicker .idx{ font-family:'Big Shoulders Display'; font-weight:800; font-size:16px; color: var(--ember); background:var(--ember-soft); padding:3px 10px; border-radius:5px; }
  .module-kicker .track{ font-size:11px; text-transform:uppercase; letter-spacing:.6px; color:var(--text-faint); }
  h2.module-title{ font-size:33px; font-weight:700; margin-bottom:14px; }
  h3{ font-size:20px; font-weight:600; margin: 30px 0 10px; color:var(--text); }
  h4{ font-family:'IBM Plex Sans'; font-size:14.5px; font-weight:700; margin: 20px 0 8px; text-transform:uppercase; letter-spacing:.3px; color:var(--text-dim);}

  .callout{ border:1px solid var(--border); background:var(--surface); border-left:3px solid var(--cyan); border-radius:8px; padding:14px 16px; margin: 18px 0; box-shadow: var(--shadow); }
  .callout.warn{ border-left-color: var(--ember); }
  .callout .cal-lbl{ font-size:11px; text-transform:uppercase; letter-spacing:.5px; font-weight:700; color:var(--cyan); margin-bottom:5px; }
  .callout.warn .cal-lbl{ color:var(--ember); }
  .callout p:last-child{ margin-bottom:0; }

  /* code blocks */
  .code{ background:#11141a; border:1px solid #262b36; border-radius:10px; margin:16px 0 22px; overflow:hidden; box-shadow: var(--shadow); }
  .code .bar{ display:flex; align-items:center; justify-content:space-between; padding:8px 14px; border-bottom:1px solid #262b36; }
  .code .bar .lang{ font-family:'IBM Plex Mono'; font-size:11px; letter-spacing:.5px; text-transform:uppercase; color:#7a8496; }
  .code .bar button{ font-family:'IBM Plex Mono'; font-size:11px; color:#9aa4b8; background:transparent; border:1px solid #2e3441; border-radius:6px; padding:4px 9px; cursor:pointer; }
  .code .bar button:hover{ color:#e7eaf0; border-color:#454d5f; }
  .code pre{ margin:0; padding:14px 16px; overflow-x:auto; }
  .code code{ color:#d7dbe4; font-size:13.6px; line-height:1.62; white-space:pre; }
  .tok-c{ color:#7a8496; font-style:italic; }
  .tok-s{ color:#e8b463; }
  .tok-k{ color:#5ec8e0; }
  .tok-f{ color:#ff9d5c; }
  .tok-n{ color:#b39dff; }

  code.inline{ background:var(--surface-2); border:1px solid var(--border); padding:1px 6px; border-radius:5px; font-size:.87em; color:var(--ember); }

  table{ width:100%; border-collapse:collapse; margin:16px 0 22px; font-size:14px; }
  th,td{ text-align:left; padding:8px 12px; border-bottom:1px solid var(--border); }
  th{ font-size:11px; text-transform:uppercase; letter-spacing:.4px; color:var(--text-faint); font-weight:600; }
  td{ color:var(--text-dim); }
  td code.inline{ font-size:12.5px; }
  .table-wrap{ overflow-x:auto; border:1px solid var(--border); border-radius:10px; background:var(--surface); }
  .table-wrap table{ margin:0; }
  .table-wrap th,.table-wrap td{ border-bottom:1px solid var(--border); }
  .table-wrap tr:last-child td{ border-bottom:none; }

  .diagram{ background:var(--surface); border:1px solid var(--border); border-radius:12px; padding:18px; margin:20px 0 26px; box-shadow:var(--shadow); }
  .diagram svg{ width:100%; height:auto; display:block; }
  .diagram .cap{ font-size:12px; color:var(--text-faint); margin-top:10px; text-align:center; }

  .keytake{ background:var(--surface-2); border-radius:10px; padding:16px 18px; margin:26px 0; }
  .keytake h4{ margin-top:0; color:var(--text); }
  .keytake ul{ margin-bottom:0; }
  .keytake li{ color:var(--text-dim); }

  .pillrow{ display:flex; flex-wrap:wrap; gap:8px; margin: 18px 0; }
  .pill{ font-size:12px; padding:5px 12px; border-radius:99px; background:var(--surface-2); border:1px solid var(--border); color:var(--text-dim); }

  .calc{ background:var(--surface); border:1px solid var(--border); border-radius:12px; padding:20px 22px; margin:22px 0 26px; box-shadow:var(--shadow); }
  .calc h4{ margin:0 0 4px; font-family:'IBM Plex Sans'; font-size:15px; font-weight:700; text-transform:none; letter-spacing:0; color:var(--text); }
  .calc-grid{ display:grid; grid-template-columns:repeat(auto-fit,minmax(148px,1fr)); gap:14px; margin:14px 0 4px; }
  .calc-field{ display:flex; flex-direction:column; gap:5px; }
  .calc-field label{ font-size:11px; text-transform:uppercase; letter-spacing:.4px; color:var(--text-faint); }
  .calc-field input{ font-family:'IBM Plex Mono'; font-size:14px; padding:8px 10px; border-radius:7px; border:1px solid var(--border); background:var(--surface-2); color:var(--text); width:100%; }
  .calc-field input:focus{ outline:2px solid var(--cyan); outline-offset:1px; }
  .calc label.chk{ display:flex; align-items:center; gap:7px; font-size:13px; color:var(--text-dim); margin-top:14px; }
  .calc-results{ display:grid; grid-template-columns:repeat(auto-fit,minmax(150px,1fr)); gap:10px; margin-top:18px; padding-top:18px; border-top:1px solid var(--border); }
  .calc-card{ background:var(--surface-2); border-radius:9px; padding:12px 14px; }
  .calc-card .v{ font-family:'Big Shoulders Display'; font-weight:700; font-size:23px; color:var(--ember); font-variant-numeric:tabular-nums; }
  .calc-card .l{ font-size:10.5px; color:var(--text-faint); text-transform:uppercase; letter-spacing:.4px; margin-top:2px; }
  .calc-note{ margin-top:16px; }
  .calc-note code.mono-block{ display:block; font-family:'IBM Plex Mono'; font-size:12.5px; color:var(--cyan); background:var(--cyan-soft); padding:10px 12px; border-radius:7px; line-height:1.7; white-space:pre-wrap; overflow-x:auto; }

  .end-note{ margin-top:70px; padding-top:30px; border-top:1px solid var(--border); text-align:center; color:var(--text-faint); font-size:13px; }

  @media (max-width: 980px){
    .sidebar{ position:fixed; left:0; top:0; transform:translateX(-100%); transition:transform .2s ease; box-shadow: 0 0 0 100vmax rgba(0,0,0,0); }
    .sidebar.open{ transform:translateX(0); box-shadow: 0 0 40px rgba(0,0,0,.35); }
    .sidebar-toggle{
      display:flex; position:fixed; top:14px; left:14px; z-index:70; width:38px; height:38px; align-items:center; justify-content:center;
      background:var(--surface); border:1px solid var(--border); border-radius:9px; box-shadow:var(--shadow); cursor:pointer; color:var(--text);
    }
    main{ padding:80px 20px 100px; }
    .hero h1{ font-size:38px; }
  }

  /* ---------- site chrome: top-level nav, non-course pages ---------- */
  .site-nav{ padding:14px 20px 16px; border-bottom:1px solid var(--border); margin-bottom:6px; display:flex; flex-direction:column; gap:8px; }
  .site-nav a{ font-size:13px; color:var(--text-dim); text-decoration:none; display:flex; align-items:center; gap:8px; }
  .site-nav a:hover{ color:var(--text); }
  .site-nav .user{ font-size:12.5px; color:var(--text-faint); }
  .site-nav .user b{ color:var(--text); font-weight:600; }

  main.page{ flex:1; min-width:0; padding:64px 40px 100px; }
  .page-wrap{ max-width:920px; margin:0 auto; }
  .page-wrap.narrow{ max-width:420px; }

  /* ---------- course catalog ---------- */
  .catalog-grid{ display:grid; grid-template-columns:repeat(auto-fill,minmax(280px,1fr)); gap:18px; margin-top:36px; }
  .course-card{ background:var(--surface); border:1px solid var(--border); border-radius:14px; padding:22px 22px 20px; box-shadow:var(--shadow); text-decoration:none; color:var(--text); display:flex; flex-direction:column; gap:10px; transition:transform .15s ease, border-color .15s ease; }
  .course-card:hover{ transform:translateY(-2px); border-color:var(--cyan); }
  .course-card .cc-emoji{ font-size:26px; }
  .course-card h3{ margin:0; font-size:22px; }
  .course-card p{ font-size:13.5px; color:var(--text-dim); margin:0; max-width:none; }
  .course-card .cc-meta{ font-size:11px; text-transform:uppercase; letter-spacing:.4px; color:var(--text-faint); margin-top:auto; padding-top:8px; display:flex; justify-content:space-between; align-items:center; }
  .course-card .cc-bar{ height:5px; border-radius:3px; background:var(--surface-2); overflow:hidden; margin-top:4px; }
  .course-card .cc-bar > i{ display:block; height:100%; background:linear-gradient(90deg, var(--cyan), var(--ember)); }

  /* ---------- buttons ---------- */
  .btn{ display:inline-flex; align-items:center; justify-content:center; gap:8px; font-family:'IBM Plex Sans'; font-weight:600; font-size:14.5px; padding:11px 20px; border-radius:9px; border:1px solid transparent; cursor:pointer; text-decoration:none; }
  .btn-primary{ background:var(--ember); color:#fff; }
  .btn-primary:hover{ filter:brightness(1.06); }
  .btn-ghost{ background:transparent; border-color:var(--border); color:var(--text); }
  .btn-ghost:hover{ background:var(--surface-2); }
  .btn-block{ width:100%; }

  /* ---------- auth forms ---------- */
  .auth-card{ background:var(--surface); border:1px solid var(--border); border-radius:14px; padding:30px 32px; box-shadow:var(--shadow); margin-top:40px; }
  .auth-card h1{ font-size:30px; margin-bottom:6px; }
  .auth-card .sub{ color:var(--text-dim); font-size:14px; margin-bottom:22px; }
  .field{ display:flex; flex-direction:column; gap:6px; margin-bottom:16px; }
  .field label{ font-size:12px; text-transform:uppercase; letter-spacing:.4px; color:var(--text-faint); }
  .field input{ font-family:'IBM Plex Sans'; font-size:15px; padding:11px 13px; border-radius:8px; border:1px solid var(--border); background:var(--surface-2); color:var(--text); }
  .field input:focus{ outline:2px solid var(--cyan); outline-offset:1px; }
  .field .hint{ font-size:11.5px; color:var(--text-faint); }
  .auth-foot{ margin-top:18px; font-size:13.5px; color:var(--text-dim); text-align:center; }
  .auth-foot a{ font-weight:600; }

  .flash{ border-radius:9px; padding:12px 15px; font-size:13.5px; margin-bottom:18px; border:1px solid; }
  .flash-error{ background:var(--ember-soft); border-color:var(--ember); color:var(--ember); }
  .flash-success{ background:var(--cyan-soft); border-color:var(--cyan); color:var(--cyan); }

  /* ---------- dashboard ---------- */
  .dash-head{ display:flex; align-items:baseline; justify-content:space-between; flex-wrap:wrap; gap:10px; margin-bottom:8px; }
  .dash-stats{ display:flex; gap:28px; margin:18px 0 8px; flex-wrap:wrap; }
  .dash-stat .v{ font-family:'Big Shoulders Display'; font-weight:800; font-size:34px; color:var(--ember); }
  .dash-stat .l{ font-size:11.5px; color:var(--text-faint); text-transform:uppercase; letter-spacing:.4px; }

  @media (max-width: 980px){
    main.page{ padding:80px 20px 100px; }
  }