@import url('https://fonts.googleapis.com/css2?family=Bricolage+Grotesque:opsz,wght@12..96,400;12..96,500;12..96,600;12..96,700;12..96,800&family=IBM+Plex+Serif:ital,wght@0,400;0,600;1,400&display=swap');

/* ======================================================================
   Kevinvariant design system
   Calm by default: a warm light page, white cards, navy ink. The three
   pigments from the logo's triangle are kept for meaning only (links,
   right/wrong, locked/free), never for decoration. Dark mode keeps the
   original indigo palette, flattened; reward themes recolour dark mode.
   ====================================================================== */
:root{
  --bg:#F6F4EF;          /* page */
  --bg-deep:#FFFFFF;     /* wells, inputs */
  --surface:#FFFFFF;     /* panels */
  --surface-2:#F0EDE6;   /* raised panels, hover */
  --line:#D9D4C8;        /* borders */
  --line-soft:#E6E2D8;
  --ink:#1B1E33;         /* text */
  --ink-soft:#4A4D62;
  --ink-faint:#6C6F84;
  --rose:#BE3358;        /* wrong, locked */
  --saffron:#986500;     /* highlights, coins */
  --aqua:#3640A8;        /* links, focus */
  --mint:#2A7A54;        /* right, free */
  --rose-wash:rgba(190,51,88,.09);
  --saffron-wash:rgba(152,101,0,.10);
  --aqua-wash:rgba(54,64,168,.08);
  --mint-wash:rgba(42,122,84,.10);
  --on-accent:#FFFFFF;   /* text on a solid pigment or ink fill */
  --btn-bg:#1B1E33; --btn-ink:#FFFFFF; --btn-hover:#30344F;
  --board:#F6F4EF; --chalk:#1B1E33; --chalk-cyan:#1F7A94; --chalk-rose:#BE3358; --chalk-gold:#986500; /* Geometry Lab ink */
  --display:system-ui,-apple-system,"Segoe UI",Roboto,"Helvetica Neue",Arial,sans-serif;
  --heading:"Bricolage Grotesque",system-ui,-apple-system,"Segoe UI",sans-serif;
  --serif:"IBM Plex Serif",Georgia,"Times New Roman",serif;
  --r-lg:14px; --r-md:10px; --r-sm:6px;
  color-scheme:light;
}
:root[data-mode="dark"]{
  --bg:#15173B; --bg-deep:#0F1130; --surface:#1D2052; --surface-2:#252964;
  --line:#343A82; --line-soft:#2A2F6E;
  --ink:#F3F1FF; --ink-soft:#BDBBE6; --ink-faint:#8A89BD;
  --rose:#FF6B8B; --saffron:#FFC53D; --aqua:#4FD8E8; --mint:#5BE3A6;
  --rose-wash:rgba(255,107,139,.14); --saffron-wash:rgba(255,197,61,.14); --aqua-wash:rgba(79,216,232,.13); --mint-wash:rgba(91,227,166,.14);
  --on-accent:#15173B;
  --btn-bg:var(--ink); --btn-ink:var(--bg); --btn-hover:var(--ink-soft);
  --board:#15173B; --chalk:#F3F1FF; --chalk-cyan:#8FD3E8; --chalk-rose:#F2A49C; --chalk-gold:#F2D98D;
  color-scheme:dark;
}

*,*::before,*::after{box-sizing:border-box}
html{-webkit-text-size-adjust:100%;scroll-padding-top:5rem}
body{margin:0;min-height:100vh;color:var(--ink);font-family:var(--display);font-size:16px;line-height:1.6;background:var(--bg)}
::selection{background:var(--aqua-wash);color:var(--ink)}
a{color:var(--aqua);text-underline-offset:3px;text-decoration-thickness:1px}
a:hover{color:var(--ink)}
:focus-visible{outline:2px solid var(--aqua);outline-offset:3px;border-radius:4px}
img{max-width:100%;height:auto;display:block}

h1,h2,h3,h4{font-family:var(--heading);color:var(--ink);line-height:1.15;letter-spacing:-.01em;margin:0 0 .5em;font-weight:600}
h1{font-size:clamp(2rem,4.4vw,2.9rem);letter-spacing:-.02em}
h2{font-size:clamp(1.4rem,2.4vw,1.8rem);margin-top:2.6rem}
h3{font-size:1.15rem}
h4{font-size:1.02rem;font-weight:600;margin-top:1.4rem}
p{max-width:68ch;margin:0 0 1em}
.lede,.lead{font-size:1.08rem;color:var(--ink-soft)}
hr{border:0;border-top:1px solid var(--line-soft);margin:2.4rem 0}
code{font-size:.92em;background:var(--bg-deep);padding:.1em .35em;border-radius:6px}

.container{max-width:74rem;margin:0 auto;padding:2.2rem 1.4rem 5rem}

/* --------------------------------------------------------------- buttons */
.btn,a.btn{
  display:inline-flex;align-items:center;gap:.5rem;font-family:var(--display);font-weight:600;font-size:.95rem;
  text-decoration:none;color:var(--btn-ink);background:var(--btn-bg);border:1px solid var(--btn-bg);
  padding:.6rem 1.1rem;border-radius:var(--r-md);margin:.25rem .4rem .25rem 0;cursor:pointer;min-height:44px;
  transition:background-color .12s ease,border-color .12s ease;
}
.btn:hover{background:var(--btn-hover);border-color:var(--btn-hover);color:var(--btn-ink)}
.btn.secondary,a.btn.secondary,.btn.ghost{background:transparent;color:var(--ink);border-color:var(--line)}
.btn.secondary:hover,.btn.ghost:hover{background:var(--surface-2);border-color:var(--ink-faint);color:var(--ink)}
.btn[disabled]{opacity:.4;cursor:default}
.btn[disabled]:hover{background:var(--btn-bg);border-color:var(--btn-bg)}
.btn.secondary[disabled]:hover,.btn.ghost[disabled]:hover{background:transparent;border-color:var(--line)}

/* ------------------------------------------------------ legacy content */
.card{background:var(--surface);border:1px solid var(--line-soft);border-radius:var(--r-lg);padding:1.5rem 1.6rem;margin:1.2rem 0}
.card h3{margin-top:0}
.board-card{border-left:4px solid var(--aqua)}

/* ----------------------------------------------------------- navigation */
.site-nav{position:sticky;top:0;z-index:50;background:var(--surface);border-bottom:1px solid var(--line-soft)}
.site-nav .bar{max-width:74rem;margin:0 auto;padding:.55rem 1.4rem;display:flex;align-items:center;gap:1.2rem}
.site-nav .brand{display:flex;align-items:center;gap:.6rem;text-decoration:none;color:var(--ink);font-family:var(--heading);font-weight:600;font-size:1.1rem;letter-spacing:-.01em;margin-right:auto}
.site-nav .brand svg{width:34px;height:34px;flex:none}
.site-nav .links{display:flex;align-items:center;gap:.2rem;list-style:none;margin:0;padding:0}
.site-nav .links>li{position:relative}
.site-nav .links a,.site-nav .links button{margin:0;font-family:var(--display);font-size:.95rem;font-weight:500;color:var(--ink-soft);text-decoration:none;background:none;border:0;padding:.5rem .75rem;border-radius:999px;cursor:pointer;display:flex;align-items:center;gap:.3rem;min-height:40px}
.site-nav .links a:hover,.site-nav .links button:hover{color:var(--ink);background:var(--surface)}
.site-nav .links a[aria-current="page"],.site-nav .links button.current{color:var(--ink);background:var(--surface-2)}
.site-nav .mode{display:flex;align-items:center;justify-content:center;width:40px;height:40px;margin:0;padding:0;border:0;border-radius:var(--r-md);background:none;color:var(--ink-soft);cursor:pointer;flex:none}
.site-nav .mode:hover{background:var(--surface-2);color:var(--ink)}
.site-nav .links{order:2}.site-nav .mode{order:3}.site-nav .toggle{order:4}
.site-nav .mode svg{width:18px;height:18px}
[data-mode-lock] .site-nav .mode{display:none}
.site-nav .links>li>a,.site-nav .links>li>button{white-space:nowrap}
@media (min-width:1021px) and (max-width:1180px){.site-nav .links .lg{display:none}}
.site-nav .caret{width:9px;height:9px;transition:transform .15s}
.site-nav button[aria-expanded="true"] .caret{transform:rotate(180deg)}
.site-nav .menu{position:absolute;top:calc(100% + 6px);left:0;min-width:17rem;background:var(--surface);border:1px solid var(--line);border-radius:var(--r-md);padding:.45rem;list-style:none;margin:0;box-shadow:0 12px 32px rgba(20,22,40,.12);display:none}
.site-nav .menu.open{display:block}
.site-nav .menu a{display:block;padding:.55rem .75rem;border-radius:10px;color:var(--ink)}
.site-nav .menu a small{display:block;color:var(--ink-faint);font-size:.8rem;font-weight:400;margin-top:.05rem}
.site-nav .menu a:hover{background:var(--surface-2)}
.site-nav .menu a[aria-current="page"]{background:var(--surface-2);box-shadow:inset 3px 0 0 var(--ink)}
.site-nav .toggle{display:none}
@media (max-width:1020px){
  .site-nav .toggle{display:flex;background:none;border:1px solid var(--line);color:var(--ink);border-radius:999px;padding:.4rem .8rem;font-family:var(--display);font-weight:600;min-height:40px;cursor:pointer}
  .site-nav .links{display:none;position:absolute;left:0;right:0;top:100%;flex-direction:column;align-items:stretch;background:var(--surface);border-bottom:1px solid var(--line);padding:.6rem 1rem 1rem;max-height:calc(100vh - 60px);max-height:calc(100dvh - 60px);overflow-y:auto;overscroll-behavior:contain}
  .site-nav.open .links{display:flex}
  .site-nav .menu{position:static;display:block;box-shadow:none;border:0;background:transparent;padding:0 0 .4rem .6rem;min-width:0}
  .site-nav .links button .caret{display:none}
  .site-nav .links button{color:var(--ink-faint);font-size:.82rem;cursor:default;padding-bottom:0}
}
/* account slot */
.site-nav .acct{margin-left:.4rem}
.site-nav .acct .signin{border:1px solid var(--line);color:var(--ink);gap:.3rem}
.site-nav .acct .signin:hover{background:var(--surface-2);color:var(--ink)}
.site-nav .acct .acct-btn{gap:.5rem;padding:.3rem .7rem .3rem .3rem;border:1px solid var(--line)}
.site-nav .acct .acct-av{width:28px;height:28px;border-radius:50%;display:grid;place-items:center;font-weight:700;font-size:.85rem;color:var(--btn-ink);background:var(--btn-bg)}
.site-nav .acct .acct-nm{max-width:9rem;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}
.site-nav .menu.right{left:auto;right:0}
.site-nav .menu .me{padding:.5rem .75rem;font-size:.82rem;color:var(--ink-faint);border-bottom:1px solid var(--line-soft);margin-bottom:.3rem;overflow:hidden;text-overflow:ellipsis}
.site-nav .menu .out{width:100%;justify-content:flex-start;color:var(--rose);padding:.55rem .75rem;border-radius:10px}
.site-nav .menu .out:hover{background:var(--rose-wash);color:var(--rose)}
@media (max-width:1020px){.site-nav .acct{order:-1;margin:0 0 .6rem;padding-bottom:.6rem;border-bottom:1px solid var(--line-soft)}.site-nav .acct .acct-btn .caret{display:none}.site-nav .acct .acct-btn{color:var(--ink);font-size:.95rem;cursor:pointer}}

/* the old flat nav, for pages not yet converted */
body>nav:not(.site-nav){background:var(--surface);border-bottom:1px solid var(--line-soft);padding:.8rem 1.4rem}
body>nav:not(.site-nav) .nav-links{display:flex;flex-wrap:wrap;gap:.3rem 1.2rem;max-width:74rem;margin:0 auto}
body>nav:not(.site-nav) .nav-links a{color:var(--ink-soft);text-decoration:none;font-weight:500}
body>nav:not(.site-nav) .nav-links a[aria-current]{color:var(--ink)}

/* Pages that have not been converted yet may draw dark ink on a canvas;
   give their drawings a light board so nothing disappears. */
body:not([data-theme-v2]) main canvas,body:not([data-theme-v2]) main svg{background:#F4F2FF;border-radius:12px}

.site-foot{max-width:74rem;margin:3rem auto 0;padding:2.2rem 1.4rem 3rem;color:var(--ink-faint);font-size:.9rem;border-top:1px solid var(--line)}
.site-foot a{color:var(--ink-soft)}

/* ======================================================================
   Shared components (redesign, October 2026)
   Section colours reuse the logo pigments, so reward themes recolour them too:
   TMUA aqua, Olympiad saffron, Practise and compete rose, Book mint.
   ====================================================================== */
[data-sec="tmua"],.sec-tmua{--accent:var(--aqua)}
[data-sec="oly"],.sec-oly{--accent:var(--saffron)}
[data-sec="play"],.sec-play{--accent:var(--rose)}
[data-sec="book"],.sec-book{--accent:var(--mint)}
[data-sec="esat"],.sec-esat{--accent:#6A3FB0}
[data-mode="dark"] [data-sec="esat"],[data-mode="dark"] .sec-esat{--accent:#B9A2FF}

.page{max-width:74rem;margin:0 auto;padding:0 1.4rem 4rem}
.page>section{margin-top:3rem}
.page>section>h2:first-child{margin-top:0}
.page .visually-hidden{position:absolute!important;width:1px;height:1px;overflow:hidden;clip:rect(0 0 0 0);white-space:nowrap}

/* hub header: a coloured rule says which section you are in */
.page .hub-head{padding:2.6rem 0 .6rem;border-bottom:1px solid var(--line-soft)}
.page .hub-head .eyebrow{display:inline-flex;align-items:center;gap:.5rem;font-weight:600;font-size:.78rem;letter-spacing:.1em;text-transform:uppercase;color:var(--ink-faint);margin-bottom:.6rem}
.page .hub-head .eyebrow::before{content:"";width:.55rem;height:.55rem;border-radius:2px;background:var(--accent,var(--ink-faint))}
.page .hub-head h1{font-size:clamp(1.9rem,4vw,2.6rem)}
.page .hub-head .lede{max-width:44rem}
.page .jump{display:flex;flex-wrap:wrap;gap:.4rem;margin:1.2rem 0 1.4rem;padding:0;list-style:none}
.page .jump a{display:inline-flex;align-items:center;min-height:36px;padding:.3rem .85rem;border:1px solid var(--line);border-radius:var(--r-md);color:var(--ink-soft);text-decoration:none;font-size:.92rem}
.page .jump a:hover{border-color:var(--ink-faint);color:var(--ink);background:var(--surface)}
.page .sub-intro{color:var(--ink-soft);max-width:62ch;margin-top:-.2rem}

/* chips: difficulty and access, the same everywhere, always linked to the explainer */
.page .chips{display:flex;flex-wrap:wrap;gap:.35rem .45rem;align-items:center;margin:.1rem 0 .7rem}
.page .chip{display:inline-flex;align-items:center;gap:.35rem;padding:.12rem .55rem;border-radius:var(--r-sm);font-size:.8rem;font-weight:500;line-height:1.5;border:1px solid var(--line);color:var(--ink-soft);background:var(--bg-deep);text-decoration:none;white-space:nowrap}
.page a.chip:hover{border-color:var(--ink-faint);color:var(--ink)}
.page .chip b{color:var(--ink);font-variant-numeric:tabular-nums}
.page .chip.free,.page .chip.open{color:var(--mint);border-color:rgba(91,227,166,.4);background:var(--mint-wash)}
.page .chip.locked{color:var(--rose);border-color:rgba(255,107,139,.4);background:var(--rose-wash)}
.page .chip.coins{color:var(--saffron);border-color:rgba(255,197,61,.4);background:var(--saffron-wash)}

/* doorway and content cards */
.page .cards{display:grid;grid-template-columns:repeat(auto-fit,minmax(16.5rem,1fr));gap:1rem;margin-top:1rem}
.page .cards.two{grid-template-columns:repeat(auto-fit,minmax(20rem,1fr))}
.page .door,.page .pc{position:relative;display:flex;flex-direction:column;background:var(--surface);border:1px solid var(--line-soft);border-top:2px solid var(--accent,var(--line-soft));border-radius:var(--r-lg);padding:1.3rem 1.3rem 1.1rem}
.page .door h3,.page .pc h3{margin:0 0 .35rem;font-size:1.18rem}
.page .door p,.page .pc p{font-size:.95rem;color:var(--ink-soft);margin:0 0 .8rem}
.page .door ul{list-style:none;margin:auto 0 0;padding:.6rem 0 0;border-top:1px solid var(--line-soft);display:grid;gap:.15rem}
.page .door ul a{display:flex;min-height:36px;align-items:center;text-decoration:none}
.page .door ul a:hover{text-decoration:underline}
.page .door .go{font-weight:700;color:var(--ink);text-decoration:none}
.page .door .go::after{content:"";position:absolute;inset:0;border-radius:inherit}
.page .door .go:hover{color:var(--aqua)}
.page .door ul{position:relative;z-index:1}
.page .pc .acts{margin-top:auto;display:flex;flex-wrap:wrap;align-items:center;gap:.3rem 1.1rem;font-size:.95rem}
.page .pc .acts .btn{margin-right:.2rem}
.page .pc .desc{font-family:var(--serif);font-size:.98rem;line-height:1.6}

/* paper and resource lists */
.page .rows{border-top:1px solid var(--line-soft);margin-top:1rem}
.page .row{display:grid;grid-template-columns:minmax(0,1.15fr) minmax(0,1fr);gap:.5rem 2rem;padding:1.15rem .2rem;border-bottom:1px solid var(--line-soft)}
.page .row h3{margin:0 0 .25rem;font-size:1.08rem}
.page .row .desc{margin:0;color:var(--ink-soft);font-family:var(--serif);font-size:.96rem;line-height:1.6}
.page .row .files{display:flex;flex-direction:column;gap:.5rem;align-content:start;font-size:.96rem}
.page .row .files .grp{display:flex;flex-wrap:wrap;align-items:center;gap:.2rem 1.1rem}
.page .row .files .grp>span{color:var(--ink-faint);min-width:4.6rem;font-size:.88rem}
.page .row .files .grp a{display:inline-flex;align-items:center;min-height:36px}
@media (max-width:760px){.page .row{grid-template-columns:1fr}.page .row .files .grp>span{flex-basis:100%;min-width:0}}

/* tool list (moved from the old homepage) */
.page .tools{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:.2rem 2.4rem;margin-top:1rem}
@media (max-width:760px){.page .tools{grid-template-columns:1fr}}
.page .tool{display:grid;grid-template-columns:2.6rem 1fr;gap:1rem;align-items:start;padding:1.05rem .8rem;border-radius:var(--r-md);text-decoration:none;color:var(--ink);border-bottom:1px solid var(--line-soft)}
.page .tool:hover{background:var(--surface);color:var(--ink)}
.page .tool svg{width:2.1rem;height:2.1rem;color:var(--ink-faint)}
.page .tool h3{margin:.05rem 0 .15rem;font-size:1.12rem}
.page .tool p{margin:0;font-size:.95rem;color:var(--ink-soft)}

/* callouts */
.page .callout{background:var(--surface);border:1px solid var(--line-soft);border-left:3px solid var(--accent,var(--ink-faint));border-radius:var(--r-md);padding:1rem 1.2rem;margin:1.2rem 0}
.page .callout p:last-child{margin-bottom:0}
.page .callout h3{margin:0 0 .3rem;font-size:1.05rem}

/* difficulty scale bar: 4.5 to 9.0 */
.page .scale{margin:1rem 0 .4rem;max-width:44rem}
.page .scale .bar{display:grid;grid-template-columns:1.5fr 1.5fr 1.6fr;height:8px;border-radius:99px;overflow:hidden}
.page .scale .bar i:nth-child(1){background:color-mix(in srgb,var(--mint) 70%,transparent)}
.page .scale .bar i:nth-child(2){background:color-mix(in srgb,var(--saffron) 70%,transparent)}
.page .scale .bar i:nth-child(3){background:color-mix(in srgb,var(--rose) 70%,transparent)}
.page .scale .ticks{display:grid;grid-template-columns:1.5fr 1.5fr 1.6fr;font-size:.84rem;color:var(--ink-soft);margin-top:.35rem}
.page .scale .ticks span b{display:block;color:var(--ink);font-size:.9rem}
.page .tiers{display:grid;grid-template-columns:repeat(auto-fit,minmax(13rem,1fr));gap:.8rem;margin:1rem 0;padding:0;list-style:none}
.page .tiers li{background:var(--surface);border:1px solid var(--line-soft);border-radius:var(--r-md);padding:.9rem 1rem;font-size:.95rem;color:var(--ink-soft)}
.page .tiers li .chip{margin-bottom:.45rem}
.page .tiers li b{color:var(--ink)}
.page table.plain{border-collapse:collapse;width:100%;font-size:.95rem;margin:1rem 0}
.page table.plain th,.page table.plain td{text-align:left;padding:.55rem .7rem .55rem 0;border-bottom:1px solid var(--line-soft);vertical-align:top}
.page table.plain th{color:var(--ink-faint);font-weight:600;font-size:.85rem}
.page table.plain td:first-child{white-space:nowrap}
.page .table-wrap{overflow-x:auto;-webkit-overflow-scrolling:touch}

/* footer, injected by site.js on every page */
.site-foot .foot-grid{display:grid;grid-template-columns:repeat(auto-fit,minmax(9.5rem,1fr));gap:1.4rem;margin-bottom:1.4rem}
.site-foot .foot-grid div{display:flex;flex-direction:column;gap:.15rem}
.site-foot .foot-grid b{color:var(--ink);font-size:.92rem;margin-bottom:.2rem}
.site-foot .foot-grid a{text-decoration:none;padding:.15rem 0}
.site-foot .foot-grid a:hover{color:var(--ink);text-decoration:underline}
.site-foot>p{margin:0;max-width:none}
@media (max-width:760px){.site-foot .foot-grid{grid-template-columns:1fr 1fr}}

/* Trap Map: the mistake behind a wrong answer (trap-map.js), shown above the solution */
.trapnote{margin:.9rem 0 .2rem;padding:.65rem .9rem;border:1px solid var(--line-soft);border-left:3px solid var(--saffron);border-radius:var(--r-sm);background:var(--saffron-wash);color:var(--ink);font-family:var(--display);font-size:.95rem;line-height:1.55;max-width:70ch}
.trapnote p{margin:0}
.trapnote .draft{margin-top:.3rem;font-size:.82rem;color:var(--ink-soft)}

/* MathJax: keep the hidden copy for screen readers inside its formula, so it never widens the page */
mjx-container{position:relative}
mjx-assistive-mml{max-width:100%;max-height:100%}
/* a block holding display maths scrolls sideways on narrow screens; part of the formula margin moves into
   padding, so the spacing is unchanged and tall formulas are never clipped */
:has(> mjx-container[display="true"]){overflow-x:auto;padding-block:.8em}
:has(> mjx-container[display="true"]) > mjx-container[display="true"]{margin-block:.2em}

/* Confidence calibration (confidence.js): Guess, Unsure or Sure before an answer is checked */
.confrow{display:flex;flex-wrap:wrap;gap:.45rem;align-items:center;margin-top:.9rem}
.confrow .conflab{flex-basis:100%;font-size:.9rem;color:var(--ink-soft)}
.confrow button .confp{font-size:.8rem;opacity:.8}

/* Solution diagrams (tkz-euclide, built by tools/build-diagrams.py). White card so the black linework reads in dark mode too. */
.sol-fig{margin:.4rem 0 1rem;padding:.6rem .8rem;background:#fff;border:1px solid var(--line);border-radius:var(--r-md);display:block;width:fit-content;max-width:100%;box-sizing:border-box}
.sol-fig img{display:block;max-width:min(100%,32rem);height:auto}