:root,[data-theme="light"]{color-scheme:light;--bg:#fff;--fg:#1a1d21;--muted:#646b75;--line:#e4e7eb;--soft:#f6f7f9;--accent:#1d4ed8;--code:#f6f7f9}
[data-theme="dark"]{color-scheme:dark;--bg:#111316;--fg:#e6e8eb;--muted:#9aa1ab;--line:#262a30;--soft:#181b20;--accent:#7aa2ff;--code:#181b20}
*{box-sizing:border-box}
html{scroll-behavior:smooth;scroll-padding-top:72px}
body{margin:0;background:var(--bg);color:var(--fg);font:16px/1.65 -apple-system,BlinkMacSystemFont,"Segoe UI",Roboto,Helvetica,Arial,sans-serif;-webkit-font-smoothing:antialiased}
a{color:var(--accent);text-decoration:none}a:hover{text-decoration:underline}
img{max-width:100%}
code,pre{font-family:ui-monospace,SFMono-Regular,Menlo,Consolas,monospace}
.site-header{position:sticky;top:0;z-index:10;background:var(--bg);border-bottom:1px solid var(--line)}
.bar{max-width:1120px;margin:0 auto;padding:0 20px;min-height:56px;display:flex;align-items:center;justify-content:space-between;gap:20px}
.brand{display:flex;align-items:center;gap:10px;font-weight:600;color:var(--fg);white-space:nowrap}.brand:hover{text-decoration:none}
.brand img{border-radius:4px}
.nav{display:flex;align-items:center;gap:4px;overflow-x:auto;scrollbar-width:none}
.nav a{color:var(--muted);font-size:14px;padding:6px 10px;border-radius:6px;white-space:nowrap}
.nav a:hover,.nav a.active{color:var(--fg);background:var(--soft);text-decoration:none}
.theme-toggle{display:inline-flex;align-items:center;justify-content:center;width:32px;height:32px;padding:0;margin-left:6px;background:transparent;border:1px solid var(--line);border-radius:6px;color:var(--muted);cursor:pointer;transition:color .15s,border-color .15s,background .15s;flex-shrink:0}
.theme-toggle:hover{color:var(--fg);background:var(--soft);border-color:var(--muted)}
.theme-toggle:focus-visible{outline:2px solid var(--accent);outline-offset:2px}
.theme-toggle svg{width:16px;height:16px;display:block}
.theme-toggle .sun-icon{display:none}
.theme-toggle .moon-icon{display:block}
[data-theme="dark"] .theme-toggle .sun-icon{display:block}
[data-theme="dark"] .theme-toggle .moon-icon{display:none}
.site-footer{border-top:1px solid var(--line);margin-top:72px;padding:28px 20px;text-align:center;color:var(--muted);font-size:14px}
/* home */
.home{max-width:820px;margin:0 auto;padding:0 20px}
.intro{padding:72px 0 40px}
.intro h1{font-size:clamp(2rem,5vw,2.75rem);line-height:1.15;margin:0 0 12px;letter-spacing:-.02em}
.intro>p{margin:0 0 28px;color:var(--muted);font-size:18px;max-width:560px}
#q{width:100%;font:inherit;padding:12px 14px;border:1px solid var(--line);border-radius:8px;background:var(--bg);color:var(--fg)}
#q:focus{outline:2px solid var(--accent);outline-offset:1px;border-color:transparent}
.intro .status{margin:10px 0 0;font-size:14px;min-height:1.4em}
.track{margin-top:48px}
.track-head{border-bottom:1px solid var(--line);padding-bottom:12px}
.track-head h2{margin:0;font-size:1.35rem}
.track-head p{margin:4px 0 6px;color:var(--muted)}
.track-head span{font-size:13px;color:var(--muted)}
.rows{list-style:none;margin:0;padding:0}
.row a{display:grid;grid-template-columns:36px 1fr auto;gap:12px;padding:14px 4px;border-bottom:1px solid var(--line);color:var(--fg);align-items:baseline}
.row a:hover{background:var(--soft);text-decoration:none}
.row .n{color:var(--muted);font-size:13px;font-variant-numeric:tabular-nums}
.row .t strong{display:block;font-weight:600}
.row .t small{display:block;color:var(--muted);font-size:14px;line-height:1.5;margin-top:2px}
.row .m{color:var(--muted);font-size:13px;white-space:nowrap}
[hidden]{display:none!important}
/* lesson layout */
.layout{max-width:1120px;margin:0 auto;padding:0 20px;display:grid;grid-template-columns:240px minmax(0,1fr);gap:56px}
.side{padding-top:32px;font-size:14px}
.side details{position:sticky;top:80px;max-height:calc(100vh - 100px);overflow:auto}
.side summary{font-weight:600;margin-bottom:10px;list-style:none;cursor:default}
.side summary::-webkit-details-marker{display:none}
.side ol{list-style:none;margin:0;padding:0;border-left:1px solid var(--line)}
.side a{display:block;padding:5px 0 5px 14px;margin-left:-1px;border-left:2px solid transparent;color:var(--muted)}
.side a:hover{color:var(--fg);text-decoration:none}
.side a.current{color:var(--fg);font-weight:600;border-left-color:var(--accent)}
.article{padding:40px 0 0;max-width:720px;min-width:0}
.kicker{margin:0 0 8px;font-size:13px;font-weight:600;text-transform:uppercase;letter-spacing:.06em}
.kicker a{color:var(--muted)}
.article h1{font-size:clamp(1.9rem,4vw,2.4rem);line-height:1.2;letter-spacing:-.02em;margin:0 0 10px}
.meta{color:var(--muted);font-size:14px;margin:0 0 20px}
.lede{font-size:1.15rem;color:var(--muted);margin:0 0 24px}
.toc{display:flex;flex-wrap:wrap;gap:6px 16px;padding:14px 0;margin:0 0 8px;border-top:1px solid var(--line);border-bottom:1px solid var(--line);font-size:14px}
.toc span{color:var(--muted);font-weight:600}
.article section{margin-top:44px}
.article h2{font-size:1.5rem;letter-spacing:-.01em;line-height:1.3;margin:0 0 14px}
.article h3{font-size:1.1rem;margin:28px 0 8px}
.article h4{font-size:1rem;margin:20px 0 6px}
.article p,.article ul,.article ol{margin:0 0 16px}
.article li{margin:4px 0}
.article li>.label{margin-right:6px}
:not(pre)>code{background:var(--code);border:1px solid var(--line);border-radius:4px;padding:1px 5px;font-size:.88em}
.label{display:inline-block;font-size:11px;font-weight:600;text-transform:uppercase;letter-spacing:.05em;color:var(--muted);border:1px solid var(--line);border-radius:4px;padding:0 6px;margin-right:6px;line-height:1.7}
.code{margin:0 0 20px;border:1px solid var(--line);border-radius:8px;background:var(--code);position:relative;overflow:hidden}
.code figcaption{font:12px ui-monospace,SFMono-Regular,Menlo,monospace;color:var(--muted);padding:8px 14px;border-bottom:1px solid var(--line)}
.code pre{margin:0;padding:14px;overflow-x:auto;font-size:13px;line-height:1.6;tab-size:2}
.code button{position:absolute;top:6px;right:8px;font:12px inherit;font-family:inherit;color:var(--muted);background:var(--bg);border:1px solid var(--line);border-radius:5px;padding:2px 8px;cursor:pointer}
.code button:hover{color:var(--fg)}
.table-wrap{overflow-x:auto;margin:0 0 20px;border:1px solid var(--line);border-radius:8px}
table{border-collapse:collapse;width:100%;font-size:14px}
th,td{text-align:left;padding:10px 14px;border-bottom:1px solid var(--line);vertical-align:top}
th{background:var(--soft);font-weight:600}
tr:last-child td{border-bottom:0}
.note{border-left:3px solid var(--accent);background:var(--soft);padding:12px 16px;margin:0 0 20px;border-radius:0 6px 6px 0}
.note>:last-child{margin-bottom:0}.note h3{margin-top:0}
.items{display:grid;grid-template-columns:repeat(auto-fill,minmax(200px,1fr));gap:10px;margin:0 0 20px}
.item{border:1px solid var(--line);border-radius:8px;padding:12px 14px;display:flex;flex-direction:column;gap:4px;font-size:14px;min-width:0}
.item span{color:var(--muted);overflow-wrap:anywhere}
.item .code{margin:6px 0 0}
.chain{list-style:none;padding:0;margin:0 0 20px}
.chain .node{display:inline-block;border:1px solid var(--line);border-radius:6px;padding:6px 12px;background:var(--soft);font-size:14px}
.chain .arrow{color:var(--muted);font-size:13px;padding:4px 12px}
.chain li{margin:0}
blockquote{margin:0 0 16px;padding-left:16px;border-left:3px solid var(--line);color:var(--muted)}
.pager{display:grid;grid-template-columns:1fr 1fr;gap:16px;margin-top:56px;padding-top:24px;border-top:1px solid var(--line)}
.pager a{display:flex;flex-direction:column;gap:2px;border:1px solid var(--line);border-radius:8px;padding:12px 16px;color:var(--fg)}
.pager a:hover{background:var(--soft);text-decoration:none}
.pager small{color:var(--muted);font-size:12px;text-transform:uppercase;letter-spacing:.05em}
.pager .next{text-align:right;grid-column:2}
.pager .prev{grid-column:1}
@media (max-width:900px){
  .layout{grid-template-columns:1fr;gap:0}
  .side{padding-top:16px}
  .side details{position:static;max-height:none;border:1px solid var(--line);border-radius:8px;padding:10px 14px}
  .side summary{cursor:pointer;margin:0}
  .side details[open] summary{margin-bottom:10px}
  .side summary::after{content:"+";float:right;color:var(--muted)}
  .side details[open] summary::after{content:"\2212"}
  .article{padding-top:28px}
  .row a{grid-template-columns:28px 1fr}
  .row .m{grid-column:2}
  .pager{grid-template-columns:1fr}.pager .next,.pager .prev{grid-column:auto}
}
@media (max-width:560px){.bar{flex-direction:column;align-items:flex-start;gap:0;padding-top:8px}.nav{width:100%}.intro{padding-top:40px}}
@media print{.site-header,.side,.pager,.site-footer,.code button{display:none}.layout{display:block}}