:root{
  --fs:17px;
  --brand:#0a68a6; --brand-d:#084f7f; --brand-l:#e6f1f9;
  --bg:#f7f9fb; --card:#ffffff; --ink:#14212b; --muted:#566573; --line:#d7e0e8;
  --ok:#137a3f; --ok-bg:#e5f5ec; --bad:#b3261e; --bad-bg:#fbe9e8; --warn-bg:#fff6e0; --warn:#7a5300;
  --shadow:0 1px 2px rgba(10,40,70,.06),0 4px 14px rgba(10,40,70,.06);
}
@media (prefers-color-scheme:dark){
  :root:not([data-theme="light"]){
    --brand:#5cb3ec; --brand-d:#8ccbf3; --brand-l:#11324a;
    --bg:#0d1419; --card:#152029; --ink:#eef3f7; --muted:#a8b7c3; --line:#2a3a47;
    --ok:#6fd99a; --ok-bg:#12321f; --bad:#ff8a80; --bad-bg:#3a1714; --warn-bg:#352a0e; --warn:#ffd77a;
    --shadow:none;
  }
}
:root[data-theme="dark"]{
  --brand:#5cb3ec; --brand-d:#8ccbf3; --brand-l:#11324a;
  --bg:#0d1419; --card:#152029; --ink:#eef3f7; --muted:#a8b7c3; --line:#2a3a47;
  --ok:#6fd99a; --ok-bg:#12321f; --bad:#ff8a80; --bad-bg:#3a1714; --warn-bg:#352a0e; --warn:#ffd77a;
  --shadow:none;
}
*{box-sizing:border-box}
html{font-size:var(--fs);-webkit-text-size-adjust:100%}
body{margin:0;background:var(--bg);color:var(--ink);font-family:system-ui,-apple-system,"Segoe UI",Roboto,"Noto Sans",Arial,sans-serif;line-height:1.55}
a{color:var(--brand)}
a:focus-visible,button:focus-visible,summary:focus-visible{outline:3px solid var(--brand);outline-offset:2px}
.wrap{max-width:1040px;margin:0 auto;padding:0 16px}
.skip{position:absolute;left:-999px}.skip:focus{left:8px;top:8px;background:var(--card);padding:8px;z-index:9}
h1{font-size:1.9rem;line-height:1.2;margin:.6em 0 .3em}
h2{font-size:1.35rem;margin:1.6em 0 .6em}
h3{font-size:1.08rem;margin:1.3em 0 .5em}
.lead{font-size:1.12rem;color:var(--muted)}
.muted{color:var(--muted)}
.katex{font-size:1.06em}
.katex-display{overflow-x:auto;overflow-y:hidden;padding:2px 0}

/* header */
.top{background:var(--card);border-bottom:1px solid var(--line);position:sticky;top:0;z-index:5}
.top-in{display:flex;align-items:center;gap:12px;min-height:56px;flex-wrap:wrap}
.brand{display:flex;align-items:center;gap:8px;text-decoration:none;color:var(--ink);font-size:1.05rem;margin-right:auto}
.logo{display:inline-grid;place-items:center;width:32px;height:32px;border-radius:8px;background:#0a68a6;color:#fff;font-weight:700}
.nav{display:flex;gap:4px;flex-wrap:wrap}
.nav a{text-decoration:none;color:var(--ink);padding:6px 10px;border-radius:8px;font-size:.95rem}
.nav a:hover,.nav a[aria-current]{background:var(--brand-l);color:var(--brand-d)}
.menu-btn{display:none}
.a11y{display:flex;gap:6px;padding-bottom:8px;flex-wrap:wrap}
.a11y button{font:inherit;font-size:.82rem;padding:4px 10px;border:1px solid var(--line);background:var(--bg);color:var(--ink);border-radius:999px;cursor:pointer;min-height:32px}
@media (max-width:760px){
  .menu-btn{display:inline-block;font:inherit;padding:6px 12px;border:1px solid var(--line);background:var(--bg);color:var(--ink);border-radius:8px}
  .nav{display:none;width:100%;padding-bottom:8px}
  .nav.open{display:flex}
  .nav a{flex:1 1 45%}
  h1{font-size:1.55rem}
}

/* hero */
.hero{background:linear-gradient(160deg,#0a68a6,#084f7f);color:#fff;padding:28px 0 36px}
.hero h1{font-size:2.3rem;color:#fff}
.hero em{font-style:normal;color:#ffd166}
.hero .lead{color:#dcecf7}
.subjects{display:flex;gap:8px;flex-wrap:wrap}
.subj{display:inline-flex;flex-direction:column;padding:6px 12px;border-radius:10px;background:rgba(255,255,255,.12);color:#fff;text-decoration:none;font-weight:600;font-size:.95rem}
.subj small{font-weight:400;font-size:.75rem;opacity:.85}
.subj.on{background:#fff;color:#0a68a6}
.subj.off{opacity:.65}
.formulas{display:flex;flex-wrap:wrap;gap:10px 22px;margin-top:22px;opacity:.9}
.formulas>span{background:rgba(255,255,255,.1);padding:6px 12px;border-radius:8px}
.btn{display:inline-block;background:#ffd166;color:#14212b;text-decoration:none;font-weight:700;padding:10px 18px;border-radius:10px;border:0;font:inherit;cursor:pointer;margin:4px 6px 4px 0}
.btn.ghost{background:transparent;color:inherit;border:2px solid currentColor}
main .btn.ghost{color:var(--brand)}
.hero .btn.ghost{color:#fff}

/* cards and levels */
.cards{display:grid;grid-template-columns:repeat(auto-fill,minmax(230px,1fr));gap:14px}
.card{background:var(--card);border:1px solid var(--line);border-radius:14px;padding:14px 16px;text-decoration:none;color:var(--ink);box-shadow:var(--shadow)}
.card h3{margin:.1em 0 .3em;color:var(--brand)}
.card p{margin:0;color:var(--muted);font-size:.95rem}
.note{background:var(--brand-l);border-left:4px solid var(--brand);padding:12px 14px;border-radius:10px;margin:18px 0}
.levels{display:grid;grid-template-columns:repeat(auto-fill,minmax(290px,1fr));gap:14px;margin-bottom:32px}
.level{display:grid;grid-template-columns:auto 1fr;gap:2px 12px;background:var(--card);border:1px solid var(--line);border-radius:14px;padding:14px 16px;text-decoration:none;color:var(--ink);box-shadow:var(--shadow)}
.lv{grid-row:span 4;display:grid;place-items:center;width:52px;height:52px;border-radius:12px;background:#0a68a6;color:#fff;font-weight:800;font-size:1.15rem}
.lt{font-weight:700}
.ls{font-size:.88rem;color:var(--brand)}
.lx{font-size:.88rem;color:var(--muted)}
.go{font-weight:700;color:var(--brand);font-size:.92rem;margin-top:4px}
.prose{max-width:780px}
.prose li{margin:.35em 0}

/* quiz */
.parts{display:flex;gap:8px;margin:14px 0}
.parts button{flex:1;font:inherit;font-weight:700;padding:10px;border-radius:12px;border:2px solid var(--line);background:var(--card);color:var(--ink);cursor:pointer}
.parts button small{display:block;font-weight:400;color:var(--muted);font-size:.8rem}
.parts button[aria-selected="true"]{border-color:var(--brand);background:var(--brand-l)}
.overall{font-weight:600;color:var(--muted)}
.units{display:grid;grid-template-columns:repeat(auto-fill,minmax(250px,1fr));gap:10px;margin-bottom:24px}
.unit-btn{display:flex;gap:10px;align-items:flex-start;text-align:left;font:inherit;padding:12px;border-radius:12px;border:1px solid var(--line);background:var(--card);color:var(--ink);cursor:pointer;box-shadow:var(--shadow)}
.unit-btn .num{flex:none;display:grid;place-items:center;width:34px;height:34px;border-radius:9px;background:var(--brand-l);color:var(--brand-d);font-weight:800}
.unit-btn .tt{display:block;font-weight:600;line-height:1.3}
.unit-btn .bs{display:block;font-size:.82rem;color:var(--muted);margin-top:3px}
.unit-btn .bs.good{color:var(--ok)}
.quiz{background:var(--card);border:1px solid var(--line);border-radius:16px;padding:16px;box-shadow:var(--shadow);margin:10px 0 24px}
.qhead{display:flex;justify-content:space-between;gap:10px;flex-wrap:wrap;font-size:.9rem;color:var(--muted)}
.qtitle{font-weight:700;color:var(--ink);font-size:1rem}
.bar{height:6px;background:var(--line);border-radius:6px;margin:8px 0 14px;overflow:hidden}
.bar i{display:block;height:100%;background:var(--brand);width:0;transition:width .3s}
.tag{display:inline-block;font-size:.72rem;font-weight:700;letter-spacing:.04em;padding:2px 8px;border-radius:999px;background:var(--brand-l);color:var(--brand-d);margin-bottom:6px}
.qtext{font-size:1.08rem;margin:4px 0 12px;overflow-x:auto}
.opts{display:grid;gap:8px}
.opt{display:flex;gap:10px;align-items:center;text-align:left;font:inherit;padding:11px 12px;border-radius:12px;border:2px solid var(--line);background:var(--bg);color:var(--ink);cursor:pointer;min-height:48px;overflow-x:auto}
.opt .k{flex:none;display:grid;place-items:center;width:28px;height:28px;border-radius:50%;background:var(--card);border:1px solid var(--line);font-weight:700;font-size:.85rem}
.opt:hover:not(:disabled){border-color:var(--brand)}
.opt:disabled{cursor:default}
.opt.right{border-color:var(--ok);background:var(--ok-bg)}
.opt.wrong{border-color:var(--bad);background:var(--bad-bg)}
.fb{margin-top:12px;padding:12px 14px;border-radius:12px;overflow-x:auto}
.fb.ok{background:var(--ok-bg);border-left:4px solid var(--ok)}
.fb.no{background:var(--bad-bg);border-left:4px solid var(--bad)}
.fb strong{display:block;margin-bottom:2px}
.qnav{display:flex;gap:8px;justify-content:flex-end;margin-top:14px;flex-wrap:wrap}
.qnav button{font:inherit;font-weight:700;padding:10px 18px;border-radius:10px;border:2px solid var(--brand);cursor:pointer;background:var(--card);color:var(--brand)}
.qnav button.primary{background:#0a68a6;border-color:#0a68a6;color:#fff}
.result{text-align:center;padding:10px 0}
.result .big{font-size:2.6rem;font-weight:800;color:var(--brand)}

/* book */
.book-tools{margin:8px 0}
.toc{background:var(--card);border:1px solid var(--line);border-radius:14px;padding:6px 18px 12px}
.toc h2{margin:.6em 0 .3em;font-size:1.1rem}
.toc ol{margin:0;padding-left:1.2em;columns:2;column-gap:28px}
@media (max-width:640px){.toc ol{columns:1}}
.unit{background:var(--card);border:1px solid var(--line);border-radius:16px;padding:4px 18px 14px;margin:22px 0;box-shadow:var(--shadow)}
.unit h2{margin:.8em 0 .4em}
.un{display:inline-block;background:#0a68a6;color:#fff;font-size:.8rem;padding:2px 9px;border-radius:999px;vertical-align:middle;margin-right:6px}
.comp{background:var(--brand-l);padding:10px 12px;border-radius:10px}
.notes li,.mis li{margin:.4em 0}
.forms{display:grid;grid-template-columns:repeat(auto-fill,minmax(260px,1fr));gap:8px}
.form{background:var(--bg);border:1px solid var(--line);border-radius:10px;padding:2px 10px;overflow-x:auto}
.mis{list-style:none;padding:0}
.mis li{background:var(--warn-bg);border-left:4px solid #e0a100;padding:8px 12px;border-radius:8px}
.ex{border:1px solid var(--line);border-radius:12px;padding:4px 14px;margin:10px 0;overflow-x:auto}
.sol{border-top:1px dashed var(--line)}
.sol-h{font-weight:700;color:var(--brand);margin:.5em 0 .2em}
.sol p{margin:.3em 0}
.practice li{margin:.8em 0}
.practice p{margin:.2em 0;overflow-x:auto}
details{background:var(--bg);border:1px solid var(--line);border-radius:10px;padding:6px 12px;margin-top:6px}
summary{cursor:pointer;color:var(--brand);font-weight:600}
.cm{color:var(--warn);font-size:.95rem}
.unit-foot{font-size:.92rem;border-top:1px solid var(--line);padding-top:8px}

.foot{border-top:1px solid var(--line);margin-top:40px;padding:18px 0;font-size:.85rem;color:var(--muted)}
@media print{
  .top,.foot,.book-tools,.unit-foot,.skip{display:none}
  body{background:#fff;color:#000}
  .unit{break-inside:auto;box-shadow:none;border:0}
  details{border:0;padding:0}
  details>summary{display:none}
  details>*{display:block}
}
/* packs */
.ticks{padding-left:1.2em}.ticks li{margin:.35em 0}
.packs{display:grid;gap:16px;grid-template-columns:repeat(auto-fill,minmax(300px,1fr))}
.pack{display:grid;grid-template-columns:110px 1fr;gap:14px;background:var(--card,#fff);border:1px solid var(--line,#d6e0ea);border-radius:14px;padding:14px}
.pack img{width:110px;height:auto;border-radius:6px;box-shadow:0 2px 8px rgba(0,0,0,.15);display:block}
.pack h3{margin:0 0 .2em}.pack ul{padding-left:1.1em;margin:.5em 0 .9em;font-size:.92em}.pack .muted{margin:0}
.previews{display:grid;gap:18px;grid-template-columns:repeat(auto-fill,minmax(280px,1fr))}
.pv{margin:0}.pv img{width:100%;height:auto;border:1px solid var(--line,#d6e0ea);border-radius:8px;box-shadow:0 2px 10px rgba(0,0,0,.12);display:block;background:#fff}
.pv figcaption{font-size:.9em;margin-top:.4em}
.buy{margin:28px 0;padding:18px;border:2px solid #0a68a6;border-radius:14px}
.buy h2{margin-top:0}.price{color:#0a68a6;font-size:1.15em}
.steps-buy{padding-left:1.3em}.steps-buy li{margin:.4em 0}
.buyform{display:grid;gap:10px;max-width:420px}
.buyform label{display:grid;gap:4px;font-weight:600;font-size:.95em}
.buyform input{font:inherit;padding:10px 12px;border:1px solid var(--line,#b9c7d5);border-radius:10px;background:var(--bg,#fff);color:inherit}
.buyform .btn{justify-self:start;border:0;cursor:pointer}
@media (max-width:420px){.pack{grid-template-columns:84px 1fr}.pack img{width:84px}}