:root{
    --bg:#0F2439;            --bg-panel:#15304A;
    --grid-line:rgba(220,234,244,0.055);   --grid-line-major:rgba(220,234,244,0.10);
    --line:#DCEAF4;          --line-dim:#7C9BB6;
    --gold:#C9A227;          --gold-soft:#E7C765;
    --white:#F5F9FC;         --rule:rgba(220,234,244,0.16);
  }
  *{ box-sizing:border-box; }
  html,body{
    margin:0; padding:0; background-color:var(--bg);
    background-image:
      linear-gradient(var(--grid-line) 1px, transparent 1px),
      linear-gradient(90deg, var(--grid-line) 1px, transparent 1px),
      linear-gradient(var(--grid-line-major) 1px, transparent 1px),
      linear-gradient(90deg, var(--grid-line-major) 1px, transparent 1px);
    background-size: 20px 20px, 20px 20px, 100px 100px, 100px 100px;
    color:var(--line); font-family:'IBM Plex Sans', sans-serif; -webkit-font-smoothing:antialiased;
  }
  .wrap{ max-width:840px; margin:0 auto; padding:56px 24px 80px; }
  .crumbs{
    display:flex; flex-wrap:wrap; gap:8px; align-items:center;
    margin:0 0 18px; padding:0;
    font-family:'IBM Plex Mono', monospace; font-size:11px;
    letter-spacing:0.08em; text-transform:uppercase; color:var(--line-dim);
  }
  .crumbs a{
    color:var(--gold-soft); text-decoration:none; border-bottom:1px solid rgba(231,199,101,0.25);
    transition:color 0.15s ease, border-color 0.15s ease;
  }
  .crumbs a:hover{color:var(--white); border-bottom-color:var(--gold-soft)}
  .crumbs span{color:var(--line-dim); opacity:0.9}
  .sheet{ position:relative; background:var(--bg-panel); border:1px solid var(--rule); padding:48px 52px 44px; }
  .sheet::before, .sheet::after{ content:""; position:absolute; width:22px; height:22px; border-color:var(--gold); border-style:solid; }
  .sheet::before{ top:-1px; left:-1px; border-width:2px 0 0 2px; }
  .sheet::after{ bottom:-1px; right:-1px; border-width:0 2px 2px 0; }
  .eyebrow{ font-family:'IBM Plex Mono', monospace; font-size:11.5px; letter-spacing:0.14em; text-transform:uppercase; color:var(--gold-soft); margin:0 0 18px; }
  h1{ font-family:'Fraunces', serif; font-style:italic; font-weight:600; font-size:38px; line-height:1.16; color:var(--white); margin:0 0 10px; }
  .subtitle{ font-size:15px; color:var(--line-dim); margin:0 0 30px; font-family:'IBM Plex Mono', monospace; }
  h2{ font-family:'Fraunces', serif; font-weight:600; font-size:21px; color:var(--white); margin:38px 0 14px; }
  h3{ font-family:'Fraunces', serif; font-weight:600; font-size:16.5px; color:var(--white); margin:26px 0 10px; }
  p{ font-size:15.5px; line-height:1.72; color:var(--line); margin:0 0 16px; }
  em{ font-style:italic; color:var(--white); }

  .objective{
    margin:8px 0 8px; padding:16px 20px; border-left:3px solid var(--gold);
    background:rgba(201,162,39,0.06); font-size:15px; line-height:1.6; color:var(--line);
  }
  .objective strong{
    display:block; font-family:'IBM Plex Mono', monospace; font-size:10.5px;
    letter-spacing:0.12em; text-transform:uppercase; color:var(--gold-soft); margin-bottom:6px;
  }

  .rule-list{ list-style:none; margin:0 0 20px; padding:0; display:grid; gap:10px; }
  .rule-list li{ font-size:15px; line-height:1.6; color:var(--line-dim); padding-left:18px; position:relative; }
  .rule-list li::before{ content:"—"; position:absolute; left:0; color:var(--gold); }
  .rule-list strong{ color:var(--line); font-weight:500; }

  table{ width:100%; border-collapse:collapse; margin:14px 0 26px; font-size:14px; }
  th, td{ text-align:left; padding:10px 12px; border-bottom:1px solid var(--rule); color:var(--line); }
  th{ font-family:'IBM Plex Mono', monospace; font-size:11px; letter-spacing:0.08em; text-transform:uppercase; color:var(--gold-soft); font-weight:500; }
  td.mono{ font-family:'IBM Plex Mono', monospace; }

  .diagram-box{
    margin:26px 0; padding:22px; background:rgba(220,234,244,0.02); border:1px solid var(--rule);
    display:flex; flex-direction:column; align-items:center;
  }
  .diagram-caption{ margin-top:12px; font-family:'IBM Plex Mono', monospace; font-size:11.5px; color:var(--line-dim); text-align:center; }
  .diagram-box svg text{ font-family:'IBM Plex Mono', monospace; }
  .diagram-box svg .fname{ font-family:'Fraunces', serif; font-style:italic; }
  .diagram-box img{ max-width:100%; width:420px; border:1px solid var(--rule); border-radius:4px; }

  .example{ margin:24px 0; border:1px solid var(--rule); }
  .example-header{
    font-family:'IBM Plex Mono', monospace; font-size:11px; letter-spacing:0.08em; text-transform:uppercase;
    color:var(--gold-soft); background:rgba(220,234,244,0.03); padding:10px 18px; border-bottom:1px solid var(--rule);
  }
  .example-body{ padding:18px 20px 20px; }
  .example-problem{
    display:flex; gap:12px; margin:0 0 16px; padding-bottom:16px; border-bottom:1px dashed var(--rule);
    font-size:15.5px; line-height:1.6; color:var(--white);
  }
  .problem-label{
    font-family:'IBM Plex Mono', monospace; font-size:11px; letter-spacing:0.08em; text-transform:uppercase;
    color:var(--gold-soft); flex-shrink:0;
  }
  .step{ display:flex; gap:12px; margin:0 0 10px; font-size:15px; line-height:1.6; color:var(--line); }
  .step-num{ font-family:'IBM Plex Mono', monospace; color:var(--gold); flex-shrink:0; }
  .result{
    margin-top:14px; padding:12px 16px; border:1px solid var(--gold);
    background:rgba(201,162,39,0.06); font-family:'IBM Plex Mono', monospace; font-size:15px; color:var(--white);
  }

  .practice-list{ list-style:none; margin:0; padding:0; display:grid; gap:14px; }
  .practice-list li{ border:1px solid var(--rule); padding:16px 18px; }
  .practice-num{ font-family:'IBM Plex Mono', monospace; color:var(--gold); margin-right:8px; }
  details.answer{ margin-top:10px; }
  details.answer summary{
    cursor:pointer; font-family:'IBM Plex Mono', monospace; font-size:11.5px; letter-spacing:0.06em;
    text-transform:uppercase; color:var(--gold-soft); outline:none;
  }
  details.answer[open] summary{ margin-bottom:8px; }
  .answer-body{ font-size:14.5px; line-height:1.65; color:var(--line-dim); padding-left:2px; }
  .answer-body strong{ color:var(--line); }

  .nav-row{
    display:flex; justify-content:space-between; align-items:center; margin-top:44px;
    padding-top:20px; border-top:1px solid var(--rule); font-family:'IBM Plex Mono', monospace; font-size:12.5px;
    gap:12px;
  }
  .nav-row a{ color:var(--line-dim); text-decoration:none; }
  .nav-row a:hover{ color:var(--gold-soft); }
  .nav-center a{ color:var(--gold-soft); }

  footer{
    margin-top:44px; padding-top:20px; border-top:1px solid var(--rule); display:flex;
    justify-content:space-between; align-items:baseline; font-family:'IBM Plex Mono', monospace;
    font-size:11px; letter-spacing:0.08em; text-transform:uppercase; color:var(--line-dim);
  }

  @media (max-width:600px){
    .sheet{ padding:36px 22px 32px; }
    h1{ font-size:28px; }
    .nav-row{ flex-wrap:wrap; }
  }
  /* feature-table variant (used by some lessons) */
  table.feature-table{ width:100%; border-collapse:collapse; margin:0 0 22px; font-size:14.5px; }
  table.feature-table th, table.feature-table td{ text-align:left; padding:10px 12px; border:1px solid var(--rule); }
  table.feature-table th{ font-family:'IBM Plex Mono', monospace; font-size:11px; letter-spacing:0.06em; text-transform:uppercase; color:var(--gold-soft); background:rgba(220,234,244,0.03); }
  table.feature-table td{ color:var(--line-dim); }
  table.feature-table td strong{ color:var(--line); font-weight:500; }
    table.feature-table{ font-size:13px; }