:root{--paper:#FBFCFE;--grid:#E3EAF6;--grid2:#CFDBF0;--ink:#1E2A60;--muted:#5B6585;--pen:#2457D6;--hi:#FFE36E;
--red:#C9302C;--green:#167A45;--line:#D7DEEA;--card:#FFFFFF;--hand:"Patrick Hand","Chalkboard SE","Comic Sans MS",cursive;
--body:"Atkinson Hyperlegible",system-ui,sans-serif}
@media (prefers-color-scheme:dark){:root:not([data-theme="light"]){--paper:#121828;--grid:#1A2236;--grid2:#212B44;--ink:#E7ECF8;
--muted:#9AA6C4;--pen:#7FA4FF;--hi:#8A7414;--red:#FF7A72;--green:#5FD08F;--line:#2A3552;--card:#18203A}}
:root[data-theme="dark"]{--paper:#121828;--grid:#1A2236;--grid2:#212B44;--ink:#E7ECF8;--muted:#9AA6C4;--pen:#7FA4FF;--hi:#8A7414;
--red:#FF7A72;--green:#5FD08F;--line:#2A3552;--card:#18203A}
*{box-sizing:border-box}
html{font-size:18px}
body{margin:0;color:var(--ink);font-family:var(--body);line-height:1.6;background-color:var(--paper);
background-image:linear-gradient(var(--grid) 1px,transparent 1px),linear-gradient(90deg,var(--grid) 1px,transparent 1px);
background-size:28px 28px}
a{color:var(--pen)}
a:focus-visible,button:focus-visible,input:focus-visible{outline:3px solid var(--pen);outline-offset:2px;border-radius:4px}
.wrap{max-width:46rem;margin:0 auto;padding:0 16px}
header.site{display:flex;align-items:center;justify-content:space-between;gap:12px;padding:18px 0 8px}
.logo{font-family:var(--hand);font-size:2rem;color:var(--ink);text-decoration:none;line-height:1}
.logo span{background:linear-gradient(transparent 55%,var(--hi) 55%,var(--hi) 92%,transparent 92%)}
nav.top a{font-family:var(--hand);font-size:1.2rem;margin-left:16px;text-decoration:none;color:var(--muted)}
h1,h2,h3,h4{font-family:var(--hand);font-weight:400;line-height:1.15;margin:1.6em 0 .4em}
h1{font-size:2.6rem;margin-top:.6em}
h2{font-size:1.9rem;color:var(--pen)}
h3{font-size:1.45rem;color:var(--pen)}
h4{font-size:1.2rem;margin:0 0 .2em}
.hl{background:linear-gradient(transparent 58%,var(--hi) 58%,var(--hi) 90%,transparent 90%);padding:0 .1em}
.lede{color:var(--muted);max-width:36rem}
.hero{padding:28px 0 8px}
.hero h1{font-size:3.4rem;margin:0}
.find{margin-top:18px;display:flex;gap:8px}
.find input{flex:1;font:inherit;font-size:1.1rem;padding:12px 14px;border:2px solid var(--ink);border-radius:10px;background:var(--card);color:var(--ink)}
.results{list-style:none;padding:0;margin:10px 0 0}
.results li a{display:flex;gap:12px;padding:8px 4px;border-bottom:1px dashed var(--line);text-decoration:none;color:var(--ink)}
.results .num,.toc .num{font-family:var(--hand);font-size:1.2rem;color:var(--pen);min-width:3.2rem}
.course-link{display:block;margin:28px 0;padding:18px 20px;border:2px solid var(--ink);border-radius:14px;background:var(--card);
text-decoration:none;color:var(--ink)}
.course-link b{font-family:var(--hand);font-weight:400;font-size:1.8rem;display:block}
.bar{height:10px;border:1.5px solid var(--ink);border-radius:6px;overflow:hidden;margin-top:8px;background:var(--card)}
.bar i{display:block;height:100%;background:var(--hi);width:0}
.unit{margin-top:30px}
.unit h2{margin-bottom:.1em}
.unit .soon{color:var(--muted);font-style:italic}
.toc{list-style:none;padding:0;margin:0}
.toc li{display:flex;align-items:baseline;gap:10px;padding:9px 2px;border-bottom:1px solid var(--line)}
.toc li a{color:var(--ink);text-decoration:none;flex:1}
.toc li a:hover{text-decoration:underline}
.toc .done{font-family:var(--hand);color:var(--green);font-size:1.2rem;min-width:1.6rem}
.crumbs{font-size:.9rem;color:var(--muted);margin-top:10px}
.crumbs a{color:var(--muted)}
.video{position:relative;aspect-ratio:16/9;border:2px solid var(--ink);border-radius:12px;overflow:hidden;background:#000;margin:18px 0}
.video iframe{position:absolute;inset:0;width:100%;height:100%;border:0}
.novideo{border:2px dashed var(--line);border-radius:12px;padding:18px;color:var(--muted);margin:18px 0}
.eq{margin:.4em 0 1.1em;overflow-x:auto;font-size:1.2rem}
.side{display:block;color:#C2650F;font-family:var(--hand);font-size:1.15rem;line-height:1.2}
@media (prefers-color-scheme:dark){:root:not([data-theme="light"]) .side{color:#FFB070}}
ol.steps{list-style:none;counter-reset:s;padding:0}
ol.steps li{counter-increment:s;display:grid;grid-template-columns:2.2rem 1fr;column-gap:.4rem;align-items:baseline;margin:.7em 0;font-size:1.1rem}
ol.steps li::before{content:counter(s);font-family:var(--hand);font-size:1.3rem;color:var(--pen);grid-column:1;grid-row:1 / span 2;
width:1.8rem;height:1.8rem;border:2px solid var(--pen);border-radius:50%;display:grid;place-items:center;align-self:start}
ol.steps li .m{grid-column:2}
ol.steps li .side{grid-column:2}
ol.steps li .m{overflow-x:auto}
ol.steps li.last .m{background:linear-gradient(transparent 20%,var(--hi) 20%,var(--hi) 85%,transparent 85%);width:fit-content;padding:0 .3em}
.mistake .wrong{color:var(--red);position:relative;display:inline-block;font-size:1.1rem}
.mistake .wrong::after{content:"";position:absolute;left:-6px;right:-6px;top:50%;border-top:3px solid var(--red);transform:rotate(-3deg)}
.mistake .right{color:var(--green);font-size:1.1rem;margin:.5em 0}
.mistake .wrong .katex{color:var(--red)}
.mistake .right .katex{color:var(--green)}
.mistake h3{color:var(--red)}
.note{padding:14px 18px;margin:20px 0;border-radius:3px;box-shadow:0 6px 14px rgba(30,42,96,.12);max-width:34rem}
.note p{margin:0}
.note.exam{background:#FFF0A8;color:#2A2A20;transform:rotate(-.6deg)}
.note.tip{background:#D7E9FF;color:#1E2A60;transform:rotate(.5deg)}
.note.exam h4{color:#B5241F}
.summary{border:2px solid var(--ink);border-radius:6px;padding:4px 20px 10px;margin:26px 0;background:var(--card)}
.summary h3{margin-top:.6em}
figure{margin:12px 0}
svg.graph{width:100%;max-width:560px;height:auto;background:var(--card);border:1.5px solid var(--line);border-radius:8px}
.g-grid{stroke:var(--grid2);stroke-width:1}
.g-axis{stroke:var(--ink);stroke-width:2}
.g-tick{fill:var(--muted);font-size:12px;font-family:var(--body)}
.g-curve{fill:none;stroke:var(--pen);stroke-width:3}
.g-area{fill:var(--pen);opacity:.18}
.g-hole{fill:var(--card);stroke:var(--red);stroke-width:2.5}
.g-pt{fill:var(--ink)}
.g-tan{stroke:#D9731A;stroke-width:2.5}
.g-lab{fill:var(--ink);font-family:var(--hand);font-size:16px}
.g-lab-t{fill:#D9731A}
table.cmp{border-collapse:collapse;width:100%;margin:8px 0 16px}
table.cmp th,table.cmp td{padding:6px 8px;border-bottom:1px solid var(--line);text-align:left}
table.cmp thead th{font-family:var(--hand);font-weight:400;font-size:1.2rem;color:var(--pen)}
.practice{margin-top:10px}
.score{font-family:var(--hand);font-size:1.3rem}
.q{padding:18px 0 22px;border-bottom:2px solid var(--line)}
.q .qtext{font-size:1.08rem}
.q .from{font-size:.85rem;color:var(--muted)}
.choices{display:grid;gap:8px;margin-top:10px}
.choices button{font:inherit;text-align:left;padding:10px 14px;border:2px solid var(--line);border-radius:10px;background:var(--card);
color:var(--ink);cursor:pointer;display:flex;gap:10px;align-items:center}
.choices button:hover{border-color:var(--pen)}
.choices button .k{font-family:var(--hand);font-size:1.2rem;color:var(--pen);min-width:1.2rem}
.choices button.right{border-color:var(--green);background:linear-gradient(transparent 0,transparent),var(--card);box-shadow:inset 0 0 0 1px var(--green)}
.choices button.wrong{border-color:var(--red);opacity:.75}
.choices button[disabled]{cursor:default}
.verdict{font-family:var(--hand);font-size:1.35rem;margin-top:8px;min-height:1.4em}
.verdict.ok{color:var(--green)}
.verdict.no{color:var(--red)}
.why{margin-top:4px;color:var(--ink)}
.retry{font:inherit;background:none;border:0;color:var(--pen);text-decoration:underline;cursor:pointer;padding:0}
.frq{padding:16px 0 24px;border-bottom:2px solid var(--line)}
.frq .part{margin:14px 0}
.frq textarea{width:100%;min-height:90px;font:inherit;padding:10px;border:2px solid var(--line);border-radius:10px;background:var(--card);color:var(--ink)}
.frq details{margin-top:8px}
.frq summary{cursor:pointer;color:var(--pen);font-weight:700}
.frq .sol{padding:10px 14px;border-left:4px solid var(--hi);margin-top:8px;background:var(--card)}
.rubric{margin:8px 0 0;padding-left:1.2rem}
.rubric label{cursor:pointer}
.usd{font-family:inherit}
.kicker{font-family:var(--hand);font-size:1.3rem;color:var(--muted);margin:1em 0 -.6em}
.note.slip{background:#FFE1DE;color:#5A1712;transform:rotate(-.4deg)}
.note.slip h4{color:#B5241F}
.tw{overflow-x:auto}
.toc li.main a{font-weight:700}
.toc small{font-weight:400;color:var(--muted)}
.unit h2 a{color:inherit;text-decoration:none}
.unit h2 a:hover{text-decoration:underline}
.subject h2{margin-top:1.4em}
.grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(15rem,1fr));gap:14px}
.grid .course-link{margin:0;padding:14px 16px}
.grid .course-link b{font-size:1.4rem;line-height:1.15}
.course-link .meta{color:var(--muted);font-size:.9rem}
.course-link .got{display:block;font-size:.8rem;color:var(--muted);min-height:1em}
.results .cn{font-family:var(--hand);font-size:1.1rem;color:var(--pen);flex:0 0 8.5rem}
.nores{color:var(--muted)}
footer{margin:60px 0 30px;color:var(--muted);font-size:.85rem;border-top:1px solid var(--line);padding-top:14px}
.theme{font:inherit;font-size:.85rem;background:none;border:1px solid var(--line);border-radius:8px;color:var(--muted);padding:4px 8px;cursor:pointer}
.askq{font:inherit;font-size:.9rem;margin-left:14px;background:none;border:0;color:var(--pen);text-decoration:underline;cursor:pointer;padding:0}
.help-open{position:fixed;right:18px;bottom:18px;z-index:20;font-family:var(--hand);font-size:1.25rem;padding:10px 18px;border-radius:999px;
border:2px solid var(--ink);background:var(--hi);color:#1E2A60;cursor:pointer;box-shadow:0 6px 16px rgba(30,42,96,.25)}
.help{position:fixed;right:18px;bottom:18px;z-index:21;width:min(400px,calc(100vw - 32px));height:min(560px,calc(100vh - 40px));display:flex;flex-direction:column;
background:var(--card);border:2px solid var(--ink);border-radius:16px;box-shadow:0 14px 40px rgba(30,42,96,.3);overflow:hidden}
.help[hidden]{display:none}
.help header{display:flex;justify-content:space-between;align-items:center;padding:10px 14px;border-bottom:1.5px solid var(--line)}
.help header b{font-family:var(--hand);font-weight:400;font-size:1.4rem}
.help-x{font-size:1.6rem;line-height:1;background:none;border:0;color:var(--muted);cursor:pointer}
.help-log{flex:1;overflow-y:auto;padding:12px 14px;display:flex;flex-direction:column;gap:10px}
.msg{flex-shrink:0;max-width:92%;padding:9px 12px;border-radius:12px;font-size:.95rem;line-height:1.5;overflow-x:auto}
.msg.bot{background:var(--paper);border:1.5px solid var(--line);align-self:flex-start}
.msg.me{background:var(--pen);color:#fff;align-self:flex-end}
.msg.err{border-color:var(--red);color:var(--red)}
.msg p{margin:.2em 0}
.help-form{border-top:1.5px solid var(--line);padding:10px 12px}
.help-form textarea{width:100%;font:inherit;font-size:.95rem;padding:8px 10px;border:2px solid var(--line);border-radius:10px;resize:none;background:var(--paper);color:var(--ink)}
.help-btns{display:flex;justify-content:flex-end;gap:8px;margin-top:6px}
.help-btns button{font:inherit;font-size:.9rem;padding:7px 14px;border-radius:999px;cursor:pointer}
.help-send{background:var(--ink);color:var(--paper);border:2px solid var(--ink)}
.help-sol{background:none;border:2px solid var(--pen);color:var(--pen)}
.help-note{margin:0;padding:0 12px 10px;font-size:.75rem;color:var(--muted)}
@media (max-width:600px){.help{right:8px;left:8px;bottom:8px;width:auto;height:min(70vh,560px)}.help-open{right:12px;bottom:12px}}
@media (max-width:600px){html{font-size:17px}.hero h1{font-size:2.6rem}h1{font-size:2.1rem}nav.top a{margin-left:10px}}
@media (prefers-reduced-motion:reduce){*{transition:none!important;animation:none!important}}