/* Layout: a squared school notebook page — one reading column with a red margin line, formulas written in ink, key ideas as margin notes */
:root{
  --paper:#f7f9fc; --grid:#dfe7f2; --ink:#1b2a4a; --muted:#56617a;
  --blue:#2446a8; --green:#1f8a55; --red:#d23c32; --card:#ffffff; --line:#cdd6e4;
  --blue-soft:#e8eefb; --green-soft:#e4f4ec; --red-soft:#fbe9e7;
  --f-display:"Bitter",Georgia,serif; --f-body:"IBM Plex Sans",system-ui,sans-serif;
  --f-math:"JetBrains Mono",ui-monospace,Consolas,monospace; --f-hand:"Caveat","Comic Sans MS",cursive;
}
@media (prefers-color-scheme:dark){:root:not([data-theme="light"]){
  --paper:#121826; --grid:#1c2536; --ink:#e6ebf5; --muted:#9aa6bd;
  --blue:#7ea2ff; --green:#4fd08e; --red:#ff7a6e; --card:#182033; --line:#2b3650;
  --blue-soft:#1d2a4d; --green-soft:#163326; --red-soft:#3a1f1f; color-scheme:dark}}
:root[data-theme="dark"]{
  --paper:#121826; --grid:#1c2536; --ink:#e6ebf5; --muted:#9aa6bd;
  --blue:#7ea2ff; --green:#4fd08e; --red:#ff7a6e; --card:#182033; --line:#2b3650;
  --blue-soft:#1d2a4d; --green-soft:#163326; --red-soft:#3a1f1f; color-scheme:dark}

*{box-sizing:border-box}
body{margin:0;background:var(--paper);color:var(--ink);font:17px/1.65 var(--f-body);
  background-image:linear-gradient(var(--grid) 1px,transparent 1px),linear-gradient(90deg,var(--grid) 1px,transparent 1px);
  background-size:24px 24px;padding-inline:16px;padding-block:24px 64px}
.page{max-width:760px;margin:0 auto;position:relative;display:flex;flex-direction:column;gap:40px}
@media (min-width:860px){.page::before{content:"";position:absolute;left:-28px;top:0;bottom:0;width:2px;background:var(--red);opacity:.55}}
h1,h2,h3{font-family:var(--f-display);text-wrap:balance;line-height:1.2;margin:0}
h1{font-size:clamp(2rem,6vw,3.1rem);font-weight:800}
h2{font-size:1.5rem;font-weight:700;display:flex;gap:12px;align-items:baseline}
h2 .n{font-family:var(--f-math);font-size:.9rem;color:var(--card);background:var(--blue);border-radius:6px;padding:2px 8px;flex:none}
p{margin:0} section{display:flex;flex-direction:column;gap:14px}
.eyebrow{font:600 .8rem var(--f-body);letter-spacing:.08em;text-transform:uppercase;color:var(--red)}
.lead{color:var(--muted);max-width:62ch}
.task{background:var(--card);border:1px solid var(--line);border-radius:10px;padding:18px 20px;display:flex;flex-direction:column;gap:8px}
.task .lt{font-size:.85rem;color:var(--muted)}
.m{font-family:var(--f-math);font-size:.95em;white-space:nowrap}
.f{color:var(--blue);font-weight:600}.g{color:var(--green);font-weight:600}.t{color:var(--red);font-weight:600}
.math{font-family:var(--f-math);font-size:1rem;background:var(--card);border:1px solid var(--line);border-radius:8px;padding:12px 16px;overflow-x:auto;line-height:1.9}
.math div{white-space:nowrap}
.math .hl{background:var(--blue-soft);border-radius:4px;padding:1px 6px}
.math .tag{color:var(--muted);font-family:var(--f-hand);font-size:1.25rem;margin-left:10px}
.note{font-family:var(--f-hand);font-size:1.45rem;line-height:1.25;color:var(--red)}
.idea{background:var(--red-soft);border:2px dashed var(--red);border-radius:12px;padding:18px 20px;display:flex;flex-direction:column;gap:10px}
.idea ol{margin:0;padding-left:22px;display:flex;flex-direction:column;gap:6px}
ul.plain{margin:0;padding-left:22px;display:flex;flex-direction:column;gap:8px}
.answer{background:var(--green-soft);border:2px solid var(--green);border-radius:12px;padding:20px;display:flex;flex-direction:column;gap:10px}
.answer .big{font-family:var(--f-math);font-size:clamp(1.2rem,4.5vw,1.6rem);font-weight:600;display:flex;flex-wrap:wrap;gap:10px 28px;color:var(--ink)}
.chips{display:flex;flex-wrap:wrap;gap:8px}
.chip{font:600 .85rem var(--f-math);background:var(--green-soft);color:var(--green);border:1px solid var(--green);border-radius:999px;padding:3px 12px}
.ok{color:var(--green);font-weight:700}
.tbl{overflow-x:auto}
table{border-collapse:collapse;width:100%;font-variant-numeric:tabular-nums;background:var(--card)}
th,td{border:1px solid var(--line);padding:8px 12px;text-align:left}
th{font-size:.85rem;color:var(--muted);font-weight:600}
td{font-family:var(--f-math);font-size:.95rem}
figure{margin:0;background:var(--card);border:1px solid var(--line);border-radius:10px;padding:10px;display:flex;flex-direction:column;gap:8px}
figure svg{width:100%;height:auto;display:block}
figcaption{font-size:.9rem;color:var(--muted);padding:0 6px 4px}
.steps{margin:0;padding:0;list-style:none;counter-reset:s;display:flex;flex-direction:column;gap:10px}
.steps li{counter-increment:s;display:flex;gap:12px;align-items:flex-start}
.steps li::before{content:counter(s);font:600 .9rem var(--f-math);color:var(--blue);border:2px solid var(--blue);border-radius:50%;width:28px;height:28px;display:grid;place-items:center;flex:none}
.steps li span{min-width:0}
.mist{display:grid;grid-template-columns:repeat(auto-fit,minmax(220px,1fr));gap:12px}
.mist div{background:var(--card);border:1px solid var(--line);border-left:4px solid var(--red);border-radius:8px;padding:12px 14px;font-size:.95rem}
details{background:var(--card);border:1px solid var(--line);border-radius:10px;padding:14px 18px}
details+details{margin-top:0}
summary{cursor:pointer;font-weight:600}
summary:focus-visible{outline:2px solid var(--blue);outline-offset:3px;border-radius:4px}
details[open] summary{margin-bottom:10px}
details .inner{display:flex;flex-direction:column;gap:10px}
.lvl{background:var(--card);border:1px solid var(--line);border-radius:12px;padding:16px 18px;display:flex;flex-direction:column;gap:12px;min-width:0}
.lvl h3{font-size:1.15rem;display:flex;flex-wrap:wrap;gap:10px;align-items:center}
.badge{font:600 .75rem var(--f-body);letter-spacing:.06em;text-transform:uppercase;color:var(--blue);background:var(--blue-soft);border-radius:999px;padding:3px 10px}
.slider{display:flex;flex-wrap:wrap;align-items:center;gap:8px 16px;padding:0 6px}
.slider input{flex:1;min-width:180px;accent-color:var(--red)}
.pic{margin:0;display:flex;flex-direction:column;gap:6px}
.pic img{width:100%;height:auto;display:block;border-radius:10px;border:1px solid var(--line);background:#fff}
.pic figcaption{font-size:.9rem;color:var(--muted)}
.duo{display:grid;grid-template-columns:repeat(auto-fit,minmax(240px,1fr));gap:12px;align-items:center}
.duo figure{min-width:0}
.minis{display:grid;grid-template-columns:repeat(auto-fit,minmax(200px,1fr));gap:12px}
.minis figcaption{font-size:.85rem}
.st-ok{color:var(--green);font-weight:600}.st-hit{color:var(--red);font-weight:700}
.foot{font-size:.85rem;color:var(--muted)}

/* ---- course additions ---- */
.topnav{display:flex;flex-wrap:wrap;gap:8px 14px;align-items:center;font-size:.9rem}
.topnav a{color:var(--blue);text-decoration:none;font-weight:600}
.topnav a:hover{text-decoration:underline}
.topnav .sep{color:var(--muted)}
.chapnav{display:grid;grid-template-columns:1fr 1fr;gap:12px}
.chapnav a{display:flex;flex-direction:column;gap:2px;background:var(--card);border:1px solid var(--line);border-radius:10px;padding:12px 16px;text-decoration:none;color:var(--ink);min-width:0}
.chapnav a small{color:var(--muted);font-size:.8rem}
.chapnav a.next{text-align:right;grid-column:2}
.chapnav a:hover{border-color:var(--blue)}
.cards{display:grid;grid-template-columns:repeat(auto-fit,minmax(230px,1fr));gap:12px}
.card{background:var(--card);border:1px solid var(--line);border-radius:12px;padding:14px 16px;display:flex;flex-direction:column;gap:6px;text-decoration:none;color:var(--ink);min-width:0}
.card:hover{border-color:var(--blue)}
.card .k{font:600 .75rem var(--f-math);color:var(--red)}
.card b{font-family:var(--f-display);font-size:1.05rem}
.card span{font-size:.9rem;color:var(--muted)}
.why{background:var(--blue-soft);border-left:4px solid var(--blue);border-radius:8px;padding:12px 16px;display:flex;flex-direction:column;gap:6px}
.why .note{color:var(--blue);font-size:1.3rem}
.ex{background:var(--card);border:1px solid var(--line);border-radius:10px;padding:14px 18px;display:flex;flex-direction:column;gap:10px}
.ex h3{font-size:1.05rem}
/* self-check task (data-ans) */
.q{background:var(--card);border:1px solid var(--line);border-radius:10px;padding:14px 16px;display:flex;flex-direction:column;gap:10px;min-width:0}
.q .qh{display:flex;gap:10px;align-items:baseline;flex-wrap:wrap}
.q .qn{font:600 .8rem var(--f-math);color:var(--card);background:var(--blue);border-radius:6px;padding:1px 8px}
.q .row{display:flex;flex-wrap:wrap;gap:8px;align-items:center}
.q input[type=text]{font:1rem var(--f-math);padding:6px 10px;border:1px solid var(--line);border-radius:8px;background:var(--paper);color:var(--ink);min-width:0;width:12em}
.q button{font:600 .9rem var(--f-body);padding:6px 14px;border-radius:8px;border:1px solid var(--blue);background:var(--blue);color:var(--card);cursor:pointer}
.q button.ghost{background:transparent;color:var(--blue)}
.q .res{font-weight:600}
.q .res.okk{color:var(--green)}.q .res.bad{color:var(--red)}
.q details{padding:10px 14px}
.stars{color:#e0a100;letter-spacing:2px}
.lvlhead{display:flex;flex-wrap:wrap;gap:10px;align-items:center}
.progress{font:600 .9rem var(--f-math);color:var(--muted)}
@media (max-width:520px){.chapnav{grid-template-columns:1fr}.chapnav a.next{grid-column:1}}
@media (max-width:600px){.math{font-size:.9rem}.math div{white-space:normal;overflow-wrap:anywhere}}