:root{
  --bg:#f4f6fb; --card:#ffffff; --ink:#0f172a; --muted:#5b6475; --line:#e3e7ef;
  --brand:#1e3a8a; --brand2:#2f55c9; --accent:#f59e0b; --ok:#15803d; --okbg:#e8f6ec;
  --bad:#b91c1c; --badbg:#fdecec; --warnbg:#fff6e5; --tipbg:#eaf1ff; --bookbg:#f3ecff; --book:#6d28d9;
  --radius:14px; --shadow:0 1px 2px rgba(15,23,42,.06),0 4px 14px rgba(15,23,42,.06);
}
@media (prefers-color-scheme:dark){:root{
  --bg:#0b1220; --card:#131c2e; --ink:#e7ecf5; --muted:#9aa5b8; --line:#24304a;
  --brand:#2f55c9; --brand2:#8aa4ff; --okbg:#0f2a1b; --ok:#4ade80; --badbg:#2d1414; --bad:#f87171;
  --warnbg:#2b2312; --tipbg:#13213d; --bookbg:#221a38; --book:#c4b5fd; --shadow:none;
}}
*{box-sizing:border-box}
html,body{margin:0;padding:0}
body{font:16px/1.55 system-ui,-apple-system,"Segoe UI",Roboto,Arial,sans-serif;background:var(--bg);color:var(--ink);
  padding-bottom:calc(68px + env(safe-area-inset-bottom));-webkit-text-size-adjust:100%}
a{color:var(--brand2)}
.topbar{position:sticky;top:0;z-index:20;display:flex;align-items:center;gap:12px;padding:10px 14px;
  padding-top:calc(10px + env(safe-area-inset-top));background:#1e3a8a;color:#fff;overflow-x:auto}
.brand{display:flex;align-items:center;gap:8px;color:#fff;text-decoration:none;font-weight:700;white-space:nowrap}
.logo{background:var(--accent);color:#1e3a8a;border-radius:8px;padding:2px 7px;font-weight:800}
.tabs{display:flex;gap:6px;margin-left:auto}
.tabs a{color:#dbe4ff;text-decoration:none;font-size:13px;padding:5px 10px;border-radius:999px;white-space:nowrap;border:1px solid rgba(255,255,255,.2)}
.tabs a.on{background:#fff;color:#1e3a8a;font-weight:700}
.tabs a .soon{font-size:10px;opacity:.8;margin-left:4px}
main{max-width:860px;margin:0 auto;padding:16px 14px 30px;outline:none}
.bottomnav{position:fixed;bottom:0;left:0;right:0;z-index:20;display:flex;background:var(--card);border-top:1px solid var(--line);
  padding-bottom:env(safe-area-inset-bottom)}
.bottomnav a{flex:1;text-align:center;padding:10px 2px 12px;font-size:12px;color:var(--muted);text-decoration:none;font-weight:600}
.bottomnav a.on{color:var(--brand2);box-shadow:inset 0 3px 0 var(--brand2)}
h1{font-size:1.5rem;line-height:1.25;margin:.2em 0 .4em}
h2{font-size:1.2rem;margin:1.2em 0 .5em}
h3{font-size:1.05rem;margin:1em 0 .4em}
.muted{color:var(--muted)} .small{font-size:.88rem}
.card{display:block;background:var(--card);border:1px solid var(--line);border-radius:var(--radius);padding:16px;margin:12px 0;box-shadow:var(--shadow)}
.grid{display:grid;gap:12px;grid-template-columns:1fr}
.grid>.card{margin:0}
@media(min-width:640px){.grid.two{grid-template-columns:1fr 1fr}.grid.three{grid-template-columns:1fr 1fr 1fr}}
.btn{display:inline-flex;align-items:center;justify-content:center;gap:6px;border:0;border-radius:10px;padding:11px 16px;font:inherit;font-weight:700;
  cursor:pointer;text-decoration:none;background:var(--brand);color:#fff;min-height:44px}
.btn.ghost{background:transparent;color:var(--brand2);border:1.5px solid var(--line)}
.btn.warn{background:var(--bad);color:#fff}
.btn.small{padding:7px 12px;min-height:36px;font-size:.9rem}
.btn[disabled]{opacity:.45;cursor:not-allowed}
.row{display:flex;gap:10px;flex-wrap:wrap;align-items:center}
.spread{display:flex;justify-content:space-between;align-items:center;gap:10px;flex-wrap:wrap}
.pill{display:inline-block;font-size:.75rem;font-weight:700;padding:2px 9px;border-radius:999px;background:var(--tipbg);color:var(--brand2)}
.pill.ok{background:var(--okbg);color:var(--ok)} .pill.bad{background:var(--badbg);color:var(--bad)}
.pill.book{background:var(--bookbg);color:var(--book)} .pill.gray{background:var(--line);color:var(--muted)}
.bar{height:9px;background:var(--line);border-radius:99px;overflow:hidden}
.bar>i{display:block;height:100%;background:linear-gradient(90deg,var(--brand2),#22c55e);border-radius:99px}
.ring{display:flex;align-items:center;gap:16px}
.ring svg{flex:0 0 auto}
.stat{font-size:1.6rem;font-weight:800}
.chlist a.chrow{display:block;text-decoration:none;color:inherit}
.chrow .num{display:inline-flex;width:30px;height:30px;border-radius:9px;background:var(--tipbg);color:var(--brand2);font-weight:800;align-items:center;justify-content:center;margin-right:10px;flex:0 0 auto}
.chrow .head{display:flex;align-items:flex-start}
.chrow .t{font-weight:700;line-height:1.3}
.disclaimer{font-size:.82rem;color:var(--muted);border-left:3px solid var(--accent);padding:6px 10px;background:var(--warnbg);border-radius:6px}
.secnav{display:flex;gap:6px;overflow-x:auto;padding:6px 0;margin-bottom:6px}
.secnav a{white-space:nowrap;font-size:.8rem;padding:5px 10px;border-radius:999px;border:1px solid var(--line);text-decoration:none;color:var(--muted)}
.secnav a.on{background:var(--brand);color:#fff;border-color:var(--brand)}
.secnav a.done::before{content:"\2713 ";color:var(--ok)}
.secnav a.on.done::before{color:#bbf7d0}
.lesson p{margin:.6em 0}
.lesson ul{padding-left:1.2em;margin:.5em 0}.lesson li{margin:.3em 0}
.tablewrap{overflow-x:auto;margin:12px 0;border:1px solid var(--line);border-radius:10px}
table{border-collapse:collapse;width:100%;font-size:.9rem;background:var(--card)}
th,td{padding:8px 10px;border-bottom:1px solid var(--line);text-align:left;vertical-align:top}
th{background:var(--tipbg);font-weight:700}
caption{caption-side:bottom;font-size:.8rem;color:var(--muted);padding:6px;text-align:left}
.callout{border-radius:12px;padding:12px 14px;margin:12px 0;border:1px solid var(--line)}
.callout .ct{font-weight:800;font-size:.8rem;text-transform:uppercase;letter-spacing:.04em;margin-bottom:4px}
.callout.tip{background:var(--tipbg)} .callout.warn{background:var(--warnbg)} .callout.book{background:var(--bookbg)}
.callout.book .ct{color:var(--book)}
.callout ul{padding-left:1.2em;margin:.3em 0}
.example{border:2px dashed var(--brand2);border-radius:12px;padding:12px 14px;margin:12px 0}
.example ol{padding-left:1.3em;margin:.4em 0}
.example .ans{font-weight:800;background:var(--okbg);color:var(--ok);padding:6px 10px;border-radius:8px;display:inline-block;margin-top:4px}
.formula{background:var(--card);border:1px solid var(--line);border-left:5px solid var(--accent);border-radius:10px;padding:10px 14px;margin:12px 0}
.formula .fx{font-family:ui-monospace,Menlo,Consolas,monospace;font-size:1rem;font-weight:700}
.visual{margin:14px 0;text-align:center} .visual svg{max-width:100%;height:auto}
.visual .cap{font-size:.8rem;color:var(--muted);margin-top:4px}
.timeline{position:relative;margin:6px 0;padding-left:26px}
.timeline::before{content:"";position:absolute;left:9px;top:6px;bottom:6px;width:3px;background:var(--brand2);border-radius:3px}
.tl-item{position:relative;margin:0 0 12px}
.tl-item::before{content:"";position:absolute;left:-23px;top:5px;width:13px;height:13px;border-radius:50%;background:var(--accent);border:3px solid var(--card)}
.tl-when{font-weight:800;color:var(--brand2);font-size:.9rem}
.tl-label{font-weight:700}
.takeaways{background:var(--okbg);border-radius:12px;padding:12px 16px;margin:16px 0}
.takeaways h3{margin-top:0;color:var(--ok)}
.q{font-size:1.05rem;font-weight:600;margin:.4em 0 .8em}
.opts{display:grid;gap:9px}
.opt{display:flex;gap:10px;align-items:flex-start;text-align:left;width:100%;padding:12px 14px;border-radius:12px;border:1.5px solid var(--line);
  background:var(--card);color:var(--ink);font:inherit;cursor:pointer;min-height:48px}
.opt .k{flex:0 0 26px;height:26px;border-radius:7px;background:var(--line);display:flex;align-items:center;justify-content:center;font-weight:800;font-size:.85rem}
.opt:hover:not([disabled]){border-color:var(--brand2)}
.opt.sel{border-color:var(--brand2);background:var(--tipbg)}
.opt.right{border-color:var(--ok);background:var(--okbg)} .opt.right .k{background:var(--ok);color:#fff}
.opt.wrong{border-color:var(--bad);background:var(--badbg)} .opt.wrong .k{background:var(--bad);color:#fff}
.opt[disabled]{cursor:default}
.feedback{margin-top:12px;border-radius:12px;padding:12px 14px;border:1px solid var(--line)}
.feedback.ok{background:var(--okbg)} .feedback.bad{background:var(--badbg)}
.feedback .verdict{font-weight:800}
.src{font-size:.85rem;margin-top:8px;color:var(--muted)} .src a{word-break:break-word}
.progress-top{display:flex;justify-content:space-between;align-items:center;font-size:.85rem;color:var(--muted);margin-bottom:6px}
.timer{font-variant-numeric:tabular-nums;font-weight:800;font-size:1.05rem;color:var(--ink)}
.timer.low{color:var(--bad)}
.navgrid{display:grid;grid-template-columns:repeat(auto-fill,minmax(40px,1fr));gap:6px}
.navgrid button{height:38px;border-radius:8px;border:1.5px solid var(--line);background:var(--card);color:var(--ink);font-weight:700;cursor:pointer;position:relative}
.navgrid button.ans{background:var(--tipbg);border-color:var(--brand2)}
.navgrid button.cur{outline:3px solid var(--accent)}
.navgrid button.flag::after{content:"";position:absolute;top:2px;right:2px;width:8px;height:8px;border-radius:50%;background:var(--accent)}
.examfoot{position:sticky;bottom:calc(60px + env(safe-area-inset-bottom));background:var(--bg);padding:8px 0;display:flex;gap:8px;justify-content:space-between}
.big{font-size:2.6rem;font-weight:900;line-height:1.1}
.pass{color:var(--ok)} .fail{color:var(--bad)}
.choice{display:flex;gap:10px;align-items:center;padding:12px;border:1.5px solid var(--line);border-radius:12px;margin:8px 0;cursor:pointer}
.choice input{width:20px;height:20px;flex:0 0 auto}
textarea{width:100%;min-height:110px;font:12px ui-monospace,monospace;border-radius:10px;border:1px solid var(--line);padding:8px;background:var(--card);color:var(--ink)}
.loading{padding:40px;text-align:center;color:var(--muted)}
.toast{position:fixed;left:50%;transform:translateX(-50%);bottom:calc(80px + env(safe-area-inset-bottom));background:#0f172a;color:#fff;padding:10px 16px;border-radius:10px;z-index:50;font-size:.9rem}
@media(min-width:900px){.bottomnav{left:50%;right:auto;width:860px;transform:translateX(-50%);border:1px solid var(--line);border-bottom:0;border-radius:12px 12px 0 0}}
@media print{.topbar,.bottomnav{display:none}}