@font-face{font-family:'Karla var';src:url(vendor/fonts/karla-latin-wght-normal.woff2) format('woff2-variations');
  font-weight:200 800;font-style:normal;font-display:swap}

/* ============ direction C — warm paper / index cards ============ */
:root{
  --paper:#f2ece1; --dot:#ddd2be; --card:#fffdf8; --card-2:#f7f2e8;
  --ink:#23201c; --ink-2:#3d3830; --muted:#7d7365; --faint:#a89c88;
  --line:#e2d8c6; --line-2:#d5c9b2;
  --teal:#0d6b63; --teal-2:#084a45; --teal-soft:#e2efec;
  --clay:#c2643a; --clay-2:#96482a; --clay-soft:#f8ebe2;
  --violet:#7c5cbf; --blue:#2f6fb0; --olive:#6b8a34;
  --lift:0 2px 0 var(--line); --lift-2:0 3px 0 var(--line);
  --kbd-bg:#f7f2e8; --kbd-line:#d5c9b2; --kbd-ink:#8b8071;
  --row:62px; --cardh:54px;
}
:root[data-theme="dark"]{
  --paper:#17140f; --dot:#2b2519; --card:#211d16; --card-2:#1b1811;
  --ink:#efe8da; --ink-2:#d4cbba; --muted:#9d9280; --faint:#7a7161;
  --line:#332c20; --line-2:#413828;
  --teal:#49b7a6; --teal-2:#2c7a6e; --teal-soft:#12302c;
  --clay:#e0895c; --clay-2:#a85a34; --clay-soft:#33211a;
  --violet:#9d84d8; --blue:#5b98d6; --olive:#93b055;
  --lift:0 2px 0 #100e0a; --lift-2:0 3px 0 #100e0a;
  --kbd-bg:#2a241a; --kbd-line:#453b2a; --kbd-ink:#a89c88;
}
@media (prefers-color-scheme:dark){:root:not([data-theme="light"]){
  --paper:#17140f; --dot:#2b2519; --card:#211d16; --card-2:#1b1811;
  --ink:#efe8da; --ink-2:#d4cbba; --muted:#9d9280; --faint:#7a7161;
  --line:#332c20; --line-2:#413828;
  --teal:#49b7a6; --teal-2:#2c7a6e; --teal-soft:#12302c;
  --clay:#e0895c; --clay-2:#a85a34; --clay-soft:#33211a;
  --violet:#9d84d8; --blue:#5b98d6; --olive:#93b055;
  --lift:0 2px 0 #100e0a; --lift-2:0 3px 0 #100e0a;
  --kbd-bg:#2a241a; --kbd-line:#453b2a; --kbd-ink:#a89c88;
}}

/* topic spines — a border, not a child element: always paints, clips cleanly
   inside the card's border-radius */
.sp-probability{--spine:var(--teal)}.sp-brainteasers{--spine:var(--violet)}
.sp-finance{--spine:var(--clay)}.sp-statistics{--spine:var(--blue)}
.sp-pure{--spine:var(--olive)}

*{box-sizing:border-box}
html,body{margin:0;height:100%}
body{
  background:var(--paper);
  background-image:radial-gradient(var(--dot) .9px,transparent .9px);
  background-size:15px 15px;
  color:var(--ink);
  font-family:'Karla var','Karla',system-ui,sans-serif;
  font-size:15px;line-height:1.6;-webkit-font-smoothing:antialiased;
  display:flex;flex-direction:column;
}
button{font:inherit;color:inherit;cursor:pointer;background:none;border:none}
a{color:var(--teal);text-decoration:none}
svg{width:16px;height:16px;fill:none;stroke:currentColor;stroke-width:2;stroke-linecap:round;stroke-linejoin:round;flex:none}
:focus-visible{outline:2px solid var(--teal);outline-offset:2px;border-radius:8px}
@media (prefers-reduced-motion:reduce){*{transition:none!important;animation:none!important}}

kbd{
  font-family:inherit;font-size:10.5px;font-weight:700;line-height:1;
  min-width:19px;height:19px;padding:0 5px;display:inline-flex;align-items:center;justify-content:center;
  color:var(--kbd-ink);background:var(--kbd-bg);border:1.5px solid var(--kbd-line);
  border-radius:6px;box-shadow:0 1.5px 0 var(--kbd-line);
}

/* ---------- header ---------- */
#hdr{
  position:sticky;top:0;z-index:30;display:grid;align-items:center;gap:16px;
  grid-template-columns:1fr minmax(0,470px) 1fr;
  padding:13px 24px;background:color-mix(in srgb,var(--paper) 88%,transparent);
  backdrop-filter:blur(12px);border-bottom:1.5px solid var(--line);
}
.brand{display:inline-flex;align-items:center;gap:10px;color:var(--ink);font-weight:700;font-size:18px;letter-spacing:-.02em;justify-self:start}
.mark{width:32px;height:32px;border-radius:11px;background:var(--teal);color:var(--card);
  display:grid;place-items:center;font-size:15px;font-weight:700;box-shadow:0 2px 0 var(--teal-2)}
.bname em{font-style:normal;color:var(--teal)}
.searchwrap{position:relative;display:flex;align-items:center}
.sicon{position:absolute;left:14px;color:var(--faint);pointer-events:none}
#search{
  width:100%;padding:10px 42px 10px 40px;font:inherit;font-size:14px;color:var(--ink);
  background:var(--card);border:1.5px solid var(--line-2);border-radius:12px;
  box-shadow:var(--lift);outline:none;transition:border-color .15s,box-shadow .15s;
}
#search::placeholder{color:var(--faint)}
#search:focus{border-color:var(--teal);box-shadow:0 2px 0 var(--teal-2)}
#search::-webkit-search-cancel-button{display:none}
.inkey{position:absolute;right:11px;pointer-events:none}
#search:focus~.inkey{opacity:0}
.hactions{display:flex;gap:8px;justify-self:end;align-items:center}
.solvedpill{display:inline-flex;align-items:baseline;gap:6px;padding:5px 12px;border-radius:999px;
  background:var(--teal-soft);color:var(--teal)}
.solvedpill b{font-size:15px;font-weight:700}
.solvedpill span{font-size:10.5px;font-weight:700;letter-spacing:.12em;text-transform:uppercase;opacity:.8}
.icon{width:36px;height:36px;display:grid;place-items:center;color:var(--muted);
  border:1.5px solid var(--line-2);border-radius:11px;background:var(--card);box-shadow:var(--lift);transition:.15s}
.icon:hover{color:var(--teal);border-color:var(--teal)}
.s-moon{display:none}
:root[data-theme="dark"] .s-sun{display:none}:root[data-theme="dark"] .s-moon{display:block}
@media (prefers-color-scheme:dark){:root:not([data-theme="light"]) .s-sun{display:none}
  :root:not([data-theme="light"]) .s-moon{display:block}}

/* ---------- layout ---------- */
main{flex:1;min-height:0;display:flex;flex-direction:column;width:100%}
.inner{width:100%;max-width:1000px;margin:0 auto;padding:0 24px}
.boot{padding:60px 0;color:var(--muted);display:flex;align-items:center;gap:12px}
.spin{width:16px;height:16px;border:2.5px solid var(--line-2);border-top-color:var(--teal);border-radius:50%;animation:sp .7s linear infinite}
@keyframes sp{to{transform:rotate(360deg)}}

/* ---------- filters ---------- */
.filters{padding:20px 0 4px;display:flex;flex-direction:column;gap:9px}
.frow{display:flex;flex-wrap:wrap;gap:8px;align-items:center}
.flabel{width:54px;flex:none;font-size:10.5px;font-weight:700;letter-spacing:.13em;text-transform:uppercase;color:var(--faint)}
.chip{display:inline-flex;align-items:center;gap:7px;padding:6px 14px;border-radius:999px;font-size:13px;font-weight:600;
  color:var(--ink-2);background:var(--card);border:1.5px solid var(--line-2);box-shadow:var(--lift);transition:.14s}
.chip:hover{border-color:var(--teal);color:var(--teal);transform:translateY(-1px)}
.chip[aria-pressed="true"]{background:var(--teal);border-color:var(--teal);color:var(--card);box-shadow:0 2px 0 var(--teal-2)}
.chip .n{font-size:11px;opacity:.65;font-variant-numeric:tabular-nums}
#f-diff .chip[aria-pressed="true"]{background:var(--clay);border-color:var(--clay);box-shadow:0 2px 0 var(--clay-2)}

.listhead{display:flex;align-items:center;gap:10px;padding:16px 2px 12px;font-size:13px;color:var(--muted)}
.count{font-weight:700;color:var(--ink);font-size:15px}
.spacer{flex:1}
.ghost{display:inline-flex;align-items:center;gap:7px;padding:6px 12px;border-radius:10px;font-size:12.5px;font-weight:600;
  color:var(--muted);border:1.5px solid transparent;transition:.14s}
.ghost svg{width:14px;height:14px}
.ghost:hover{color:var(--teal);background:var(--card);border-color:var(--line-2)}

/* ---------- list ---------- */
.scroller{flex:1;min-height:0;overflow-y:auto;position:relative;contain:strict;scrollbar-gutter:stable}
#sizer{position:absolute;top:0;left:0;width:1px;opacity:0}
#rows{position:absolute;top:0;left:0;right:0}
.row{position:absolute;left:0;right:0;height:var(--row);padding:0 24px 8px;
  display:flex;justify-content:center}
.card{
  width:100%;max-width:952px;
  height:var(--cardh);display:flex;align-items:stretch;background:var(--card);
  border:1.5px solid var(--line-2);border-left:7px solid var(--spine,var(--line-2));
  border-radius:14px;box-shadow:var(--lift);
  cursor:pointer;transition:transform .12s,box-shadow .12s,border-color .12s;
}
.card:hover{transform:translateY(-2px);box-shadow:0 4px 0 var(--line-2);border-color:var(--line-2)}
.cbody{flex:1;min-width:0;display:flex;align-items:center;gap:15px;padding:0 18px}
.card .id{width:38px;flex:none;font-size:13px;font-weight:700;color:var(--faint);font-variant-numeric:tabular-nums}
.card.done .id{color:var(--teal)}
.card .t{flex:1;min-width:0;font-size:16.5px;font-weight:600;letter-spacing:-.01em;color:var(--ink);
  white-space:nowrap;overflow:hidden;text-overflow:ellipsis}
.card.done .t{color:var(--muted)}
.lvl{font-size:10.5px;font-weight:700;letter-spacing:.12em;text-transform:uppercase;flex:none}
.l-easy{color:var(--teal)}.l-medium{color:var(--clay)}.l-hard{color:var(--clay)}
.dot{width:26px;height:26px;border-radius:50%;flex:none;display:grid;place-items:center;
  border:1.5px solid var(--line-2);color:transparent}
.dot.done{background:var(--teal);border-color:var(--teal);color:var(--card)}
.dot.mark{border-color:var(--clay);color:var(--clay)}
.dot svg{width:14px;height:14px}
.empty{padding:56px 6px;color:var(--muted)}

/* ---------- question ---------- */
.qwrap{flex:1;min-height:0;overflow-y:auto;scrollbar-gutter:stable}
.qpage{width:100%;max-width:1000px;margin:0 auto;padding:0 24px 60px}
.qnav{display:flex;align-items:center;gap:12px;padding:16px 0;font-size:13px;flex-wrap:wrap}
.back{display:inline-flex;align-items:center;gap:6px;color:var(--muted);font-weight:600}
.back:hover{color:var(--teal)}
.ctx{display:inline-flex;align-items:center;gap:3px;background:var(--card-2);border:1.5px solid var(--line-2);
  color:var(--ink-2);border-radius:999px;padding:4px 5px 4px 13px;font-size:12.5px;font-weight:700}
.ctx a{color:inherit}
.ctx .x{display:grid;place-items:center;width:21px;height:21px;border-radius:50%;color:var(--muted)}
.ctx .x svg{width:12px;height:12px}
.ctx .x:hover{color:var(--clay);background:var(--clay-soft)}
.pager{display:flex;align-items:center;gap:6px}
.pgbtn{display:inline-flex;align-items:center;gap:7px;padding:6px 11px;border-radius:11px;color:var(--muted);
  border:1.5px solid var(--line-2);background:var(--card);box-shadow:var(--lift);font-weight:600;transition:.14s}
.pgbtn .mo{display:none}
.pgbtn span{font-size:12.5px}
.pgbtn:hover:not(:disabled){color:var(--teal);border-color:var(--teal)}
.pgbtn:disabled{opacity:.4;cursor:not-allowed}
.pos{font-size:13px;color:var(--muted);font-variant-numeric:tabular-nums;min-width:70px;text-align:center;font-weight:700}

.qcard{background:var(--card);border:1.5px solid var(--line-2);border-radius:20px;
  box-shadow:var(--lift-2);overflow:hidden;max-width:770px;width:100%;margin:0 auto}
.qtop{height:7px;background:var(--spine,var(--teal))}
.qinner{padding:26px 32px 30px}
.qmeta{display:flex;align-items:center;gap:11px;margin-bottom:11px}
.qmeta .lab{font-size:10.5px;font-weight:700;letter-spacing:.13em;text-transform:uppercase}
.qmeta .sep{width:4px;height:4px;border-radius:50%;background:var(--line-2)}
#qtitle{font-size:31px;font-weight:700;letter-spacing:-.028em;line-height:1.15;margin:0 0 24px;color:var(--ink)}
.tabbar{display:flex;align-items:center;gap:10px;margin-bottom:24px;flex-wrap:wrap}
.segs{display:inline-flex;gap:4px;padding:4px;background:var(--card-2);border:1.5px solid var(--line);border-radius:13px}
.seg{display:inline-flex;align-items:center;gap:8px;padding:7px 16px;border-radius:9px;font-size:13.5px;font-weight:700;color:var(--muted);transition:.14s}
.seg:hover{color:var(--ink)}
.seg.on{background:var(--card);color:var(--ink);box-shadow:var(--lift)}
.act{display:inline-flex;align-items:center;gap:8px;padding:7px 13px;border-radius:11px;font-size:12.5px;font-weight:600;
  color:var(--muted);border:1.5px solid var(--line-2);background:var(--card);box-shadow:var(--lift);transition:.14s}
.act:hover{color:var(--teal);border-color:var(--teal)}
.act.on.done{background:var(--teal);border-color:var(--teal);color:var(--card);box-shadow:0 2px 0 var(--teal-2)}
.act.on.bk{background:var(--clay);border-color:var(--clay);color:var(--card);box-shadow:0 2px 0 var(--clay-2);}
.act.on.bk svg{fill:currentColor}

.pane{font-size:16.5px;line-height:1.8;color:var(--ink-2);min-height:120px}
.pane p{margin:0 0 1.15em}.pane>p:last-child{margin-bottom:0}
.pane .katex{font-size:1.02em;color:var(--ink)}
.katex-display{margin:1.4em 0;overflow-x:auto;overflow-y:hidden;padding:4px 0}
.pane table{border-collapse:collapse;margin:1.2em 0;font-size:14px}
.pane td{border:1.5px solid var(--line);padding:5px 12px;text-align:center}
.skel{color:var(--faint);font-size:14px}

/* ---------- answer box ---------- */
.ansbox[hidden]{display:none}
.ansbox{margin-top:26px;padding-top:22px;border-top:1.5px dashed var(--line-2)}
.anslabel{font-size:10.5px;font-weight:700;letter-spacing:.13em;text-transform:uppercase;color:var(--faint);margin-bottom:10px}
.ansrow{display:flex;gap:10px;align-items:stretch;flex-wrap:wrap}
#ansin{flex:1;min-width:180px;padding:11px 16px;font:inherit;font-size:15px;color:var(--ink);
  background:var(--card-2);border:1.5px solid var(--line-2);border-radius:12px;outline:none;transition:.15s}
#ansin::placeholder{color:var(--faint)}
#ansin:focus{border-color:var(--teal);background:var(--card)}
.btn-go{display:inline-flex;align-items:center;gap:9px;padding:11px 22px;border-radius:12px;
  background:var(--teal);color:var(--card);font-size:14px;font-weight:700;box-shadow:0 2px 0 var(--teal-2);transition:.14s}
.btn-go:hover{filter:brightness(1.06)}
.btn-go kbd{background:rgba(255,255,255,.16);border-color:rgba(255,255,255,.3);color:inherit;box-shadow:none}
.ansmsg{min-height:22px;margin-top:11px;font-size:14px;font-weight:600;display:flex;align-items:center;gap:8px}
.ansmsg.ok{color:var(--teal)}
.ansmsg.no{color:var(--clay)}
.ansbox.ok #ansin{border-color:var(--teal);background:var(--teal-soft)}
.ansbox.no #ansin{border-color:var(--clay)}
.ansmsg .reveal{font-weight:600;color:var(--muted);text-decoration:underline;cursor:pointer}
@keyframes nudge{0%,100%{transform:translateX(0)}25%{transform:translateX(-4px)}75%{transform:translateX(4px)}}
.ansbox.no .ansrow{animation:nudge .22s ease}

/* ---------- scrollbars: native position, fade in while scrolling ---------- */
.scroller,.qwrap{scrollbar-width:thin;scrollbar-color:transparent transparent;
  transition:scrollbar-color .3s ease}
.scroller:hover,.qwrap:hover,.scroller.sc,.qwrap.sc{scrollbar-color:var(--line-2) transparent}
.scroller::-webkit-scrollbar,.qwrap::-webkit-scrollbar{width:12px}
.scroller::-webkit-scrollbar-track,.qwrap::-webkit-scrollbar-track{background:transparent}
.scroller::-webkit-scrollbar-thumb,.qwrap::-webkit-scrollbar-thumb{
  background-color:transparent;border-radius:99px;border:3.5px solid transparent;
  background-clip:content-box;transition:background-color .3s ease}
.scroller:hover::-webkit-scrollbar-thumb,.qwrap:hover::-webkit-scrollbar-thumb,
.scroller.sc::-webkit-scrollbar-thumb,.qwrap.sc::-webkit-scrollbar-thumb{
  background-color:var(--line-2);background-clip:content-box}

/* ---------- shortcuts ---------- */
.sheet[hidden]{display:none}
.sheet{position:fixed;inset:0;z-index:60;display:grid;place-items:center;padding:20px;
  background:rgba(35,32,28,.45);backdrop-filter:blur(3px);animation:fade .14s ease}
@keyframes fade{from{opacity:0}}
.sheetcard{width:min(410px,100%);background:var(--card);border:1.5px solid var(--line-2);
  border-radius:20px;box-shadow:0 14px 40px rgba(35,32,28,.28);padding:22px 24px 24px;animation:pop .17s cubic-bezier(.2,.8,.3,1)}
@keyframes pop{from{opacity:0;transform:translateY(9px) scale(.98)}}
.sheethead{display:flex;align-items:center;margin-bottom:16px}
.sheethead h2{font-size:16px;font-weight:700;letter-spacing:-.015em;margin:0;flex:1}
.sheetcard dl{display:grid;grid-template-columns:auto 1fr;gap:12px 18px;margin:0;align-items:center}
.sheetcard dt{display:flex;gap:5px}
.sheetcard dd{margin:0;font-size:13.5px;color:var(--muted);font-weight:500}

/* ---------- mobile ---------- */
@media (max-width:760px){
  kbd,.inkey{display:none!important}
  #hdr{grid-template-columns:auto 1fr auto;gap:10px;padding:10px 14px}
  .bname{display:none}
  .solvedpill{display:none}
  .inner{padding:0 14px}
  .row{padding:0 14px 8px}
  .qpage{padding:0 14px 48px}
  .flabel{display:none}
  .cbody{gap:11px;padding:0 14px}
  .card .id{width:30px;font-size:12px}
  .card .t{font-size:15.5px}
  .lvl{display:none}
  .qinner{padding:22px 20px 26px}
  #qtitle{font-size:24px;margin-bottom:20px}
  .pane{font-size:16px}
  .pgbtn .mo{display:block}.pgbtn span{display:none}
  .pos{min-width:58px;font-size:12.5px}
  #exp,#imp{display:none}
  .qcard{border-radius:16px}
}