/* RL Zero-to-Hero Academy — custom CSS, no frameworks. Palette: paper white,
   slate ink, deep teal (primary), amber (highlight). Serif headings, sans body. */
:root{
  --ink:#1c1917; --muted:#57534e; --faint:#78716c;
  --line:#e5e1d8; --panel:#f5f4f0; --card:#ffffff;
  --teal:#0e6b5e; --teal-dark:#0a4f45; --teal-soft:#e2efec;
  --amber:#b45309; --amber-soft:#fbf1e2;
  --good:#15803d; --bad:#b91c1c;
  --radius:10px; --maxw:880px;
  --serif:"Palatino Linotype",Palatino,Cambria,"Book Antiqua",Georgia,serif;
  --sans:system-ui,-apple-system,"Segoe UI",Roboto,"Helvetica Neue",Arial,sans-serif;
  --mono:ui-monospace,SFMono-Regular,Menlo,Consolas,monospace;
}
*{box-sizing:border-box}
html{scroll-behavior:smooth}
body{margin:0;background:var(--card);color:var(--ink);font-family:var(--serif);font-size:17px;line-height:1.65}
.topbar,.sidebar,.overlay-card{font-family:var(--sans);font-size:16px;line-height:1.6}
h1,h2,h3{font-family:var(--serif);line-height:1.25;color:var(--ink)}
a{color:var(--teal)}
kbd{font-family:var(--mono);font-size:12px;background:var(--panel);border:1px solid var(--line);border-bottom-width:2px;border-radius:5px;padding:0 5px}
/* ---- top bar ---- */
.topbar{display:flex;align-items:center;gap:14px;padding:10px 18px;border-bottom:1px solid var(--line);position:sticky;top:0;background:var(--card);z-index:20}
.brand{display:flex;align-items:center;gap:10px;text-decoration:none;color:inherit;white-space:nowrap}
.brand-text{display:flex;flex-direction:column;line-height:1.2}
.brand-text strong{font-family:var(--serif);font-size:18px}
.brand-text small{color:var(--faint);font-size:12px}
.topbar-center{flex:1;display:flex;justify-content:center}
#search{width:min(440px,100%);padding:8px 14px;border:1px solid var(--line);border-radius:999px;font-size:14px;background:var(--panel)}
#search:focus{outline:2px solid var(--teal);background:#fff}
.topbar-actions{display:flex;align-items:center;gap:10px}
.mode-toggle{display:flex;border:1px solid var(--line);border-radius:999px;overflow:hidden}
.mode-toggle button{border:0;background:transparent;padding:7px 14px;font-size:13px;font-weight:600;color:var(--muted);cursor:pointer}
.mode-toggle button.active{background:var(--teal);color:#fff}
.btn-amber{background:var(--amber);color:#fff;border:0;border-radius:999px;padding:8px 16px;font-size:13px;font-weight:700;cursor:pointer}
.btn-amber:hover{background:#92400e}
/* ---- layout ---- */
.layout{display:flex;align-items:stretch;min-height:calc(100vh - 59px)}
.sidebar{width:232px;flex-shrink:0;border-right:1px solid var(--line);background:var(--panel);padding:14px 12px;position:sticky;top:59px;max-height:calc(100vh - 59px);overflow:auto}
.nav-toggle{border:1px solid var(--line);background:var(--panel);border-radius:8px;font-size:16px;line-height:1;padding:7px 11px;cursor:pointer;color:var(--muted)}
.nav-toggle:hover{border-color:var(--teal);color:var(--teal-dark)}
body.nav-collapsed .sidebar{display:none}
.sidebar-head{display:flex;justify-content:space-between;align-items:center;font-size:12px;font-weight:700;letter-spacing:.06em;text-transform:uppercase;color:var(--faint);padding:0 8px 8px}
.sidebar-head button{background:none;border:0;color:var(--faint);cursor:pointer;font-size:12px;text-decoration:underline}
.track{font-size:11px;font-weight:700;letter-spacing:.08em;text-transform:uppercase;color:var(--teal-dark);padding:12px 8px 4px}
.navlink{display:flex;gap:9px;align-items:flex-start;text-decoration:none;color:inherit;padding:7px 8px;border-radius:8px;font-size:14px}
.navlink:hover{background:#e9e7e0}
.navlink.active{background:var(--teal-soft);font-weight:600}
.navlink .no{font-family:var(--mono);font-size:11px;color:var(--faint);padding-top:3px;min-width:22px}
.navlink .dot{margin-left:auto;flex-shrink:0;width:9px;height:9px;border-radius:50%;background:#d6d0c2;margin-top:7px}
.navlink.done .dot{background:var(--good)}
.sidebar-foot{font-size:12px;color:var(--faint);padding:14px 8px 4px}
.content{flex:1;min-width:0;padding:30px 34px 90px}
.content-inner{max-width:var(--maxw);margin:0 auto}
/* ---- topic page ---- */
.crumb{font-size:13px;color:var(--faint);margin-bottom:6px}
.crumb a{text-decoration:none}
.topic-title{font-size:38px;margin:0 0 4px}
.tagline{font-size:17px;color:var(--muted);margin:0 0 10px}
.meta-row{display:flex;flex-wrap:wrap;gap:8px;margin:12px 0 20px}
.chip{font-size:12.5px;background:var(--panel);border:1px solid var(--line);border-radius:999px;padding:3px 12px;color:var(--muted)}
.chip.teal{background:var(--teal-soft);border-color:#c4d9d3;color:var(--teal-dark);font-weight:600}
.outcomes{background:var(--panel);border:1px solid var(--line);border-radius:var(--radius);padding:12px 18px;margin:0 0 8px}
.outcomes h3{margin:6px 0;font-size:16px;font-family:var(--sans)}
.outcomes ul{margin:6px 0;padding-left:20px}
.section{margin-top:30px}
.section > h2{font-size:25px;border-bottom:2px solid var(--teal);padding-bottom:6px;margin-bottom:12px}
.section h3{font-size:19px;margin:18px 0 6px}
.section.quick-only{display:none}
body.mode-quick .section.deep-only{display:none}
body.mode-quick .section.quick-only{display:block}
/* formula blocks */
.formula{background:#101c1a;color:#f2f5f3;border-radius:var(--radius);padding:12px 18px;margin:12px 0;overflow-x:auto;font-family:var(--mono);font-size:15px}
.formula .flabel{display:block;font-family:var(--sans);font-size:12px;color:#9fd0c4;letter-spacing:.04em;margin-bottom:6px}
.formula .where{font-family:var(--sans);font-size:13px;color:#c8d4d0;margin-top:8px}
.frac{display:inline-block;vertical-align:middle;text-align:center;margin:0 2px}
.frac .num{display:block;border-bottom:1px solid currentColor;padding:0 6px;line-height:1.35}
.frac .den{display:block;padding:0 6px;line-height:1.35}
sup,sub{font-size:.68em}
/* callouts, lists */
.callout{border:1px solid var(--line);border-left:4px solid var(--teal);background:var(--panel);border-radius:0 10px 10px 0;padding:10px 16px;margin:12px 0}
.callout.warn{border-left-color:var(--amber);background:var(--amber-soft)}
.callout.bad{border-left-color:var(--bad)}
.grid2{display:grid;grid-template-columns:1fr 1fr;gap:12px}
@media(max-width:760px){.grid2{grid-template-columns:1fr}}
.minicard{border:1px solid var(--line);border-radius:var(--radius);padding:12px 15px;background:var(--card)}
.minicard h4{margin:0 0 4px;font-size:15px}
.minicard p{margin:4px 0;font-size:14px;color:var(--muted)}
.diagram{border:1px solid var(--line);border-radius:var(--radius);padding:10px;background:var(--card);margin:12px 0}
.diagram svg{width:100%;height:auto;display:block}
.diagram figcaption{font-size:13px;color:var(--faint);padding:6px 4px 2px}
.steps{counter-reset:step;list-style:none;padding:0}
.steps li{counter-increment:step;position:relative;padding:8px 0 8px 44px;border-bottom:1px dashed var(--line)}
.steps li:last-child{border-bottom:0}
.steps li::before{content:counter(step);position:absolute;left:0;top:8px;width:28px;height:28px;border-radius:50%;background:var(--teal);color:#fff;font-weight:700;font-size:14px;display:flex;align-items:center;justify-content:center}
/* labs */
.lab{border:1px solid #c4d9d3;border-radius:var(--radius);background:#fbfdfc;margin:14px 0;overflow:hidden}
.lab-head{background:var(--teal-soft);padding:10px 16px;display:flex;gap:10px;align-items:center}
.lab-head .beaker{font-weight:700;color:var(--teal-dark);font-size:13px;letter-spacing:.05em;text-transform:uppercase}
.lab-head h3{margin:0;font-size:17px;font-family:var(--sans)}
.lab-body{padding:14px 16px}
.lab-body p.desc{margin-top:0;color:var(--muted);font-size:14.5px}
.controls{display:flex;flex-wrap:wrap;gap:12px 18px;margin:10px 0;align-items:flex-end}
.ctl{display:flex;flex-direction:column;gap:2px;font-size:13px}
.ctl label{font-weight:600;color:var(--muted)}
.ctl output{font-family:var(--mono);font-weight:700;color:var(--ink)}
.ctl input[type=range]{width:170px;accent-color:var(--teal)}
.ctl select{padding:5px 8px;border:1px solid var(--line);border-radius:6px;font-size:13px}
.btnrow{display:flex;flex-wrap:wrap;gap:8px;margin:10px 0}
button.btn{background:var(--teal);color:#fff;border:0;border-radius:8px;padding:8px 16px;font-size:13.5px;font-weight:600;cursor:pointer}
button.btn:hover{background:var(--teal-dark)}
button.btn.ghost{background:#fff;color:var(--teal-dark);border:1px solid var(--teal)}
button.btn:disabled{opacity:.5;cursor:default}
canvas.chart{width:100%;height:240px;border:1px solid var(--line);border-radius:8px;background:#fff}
canvas.grid{border:1px solid var(--line);border-radius:8px;background:#fff;max-width:100%}
.lab-flex{display:flex;flex-wrap:wrap;gap:16px;align-items:flex-start}
.readout{font-family:var(--mono);font-size:13px;background:#101c1a;color:#d9efe9;border-radius:8px;padding:10px 14px;margin-top:10px;white-space:pre-wrap}
.legend{display:flex;flex-wrap:wrap;gap:12px;font-size:13px;margin-top:6px}
.legend i{display:inline-block;width:14px;height:4px;border-radius:2px;margin-right:5px;vertical-align:middle}
/* quiz + flashcards + checklist */
.quiz-q{border:1px solid var(--line);border-radius:var(--radius);padding:12px 16px;margin:10px 0}
.quiz-q p.q{font-weight:600;margin:0 0 8px}
.quiz-q .opt{display:block;width:100%;text-align:left;background:#fff;border:1px solid var(--line);border-radius:8px;padding:8px 12px;margin:6px 0;cursor:pointer;font-size:14.5px}
.quiz-q .opt:hover:not(:disabled){border-color:var(--teal)}
.quiz-q .opt.correct{background:#e7f6ec;border-color:var(--good)}
.quiz-q .opt.wrong{background:#fdecec;border-color:var(--bad)}
.quiz-q .why{font-size:13.5px;color:var(--muted);margin:8px 0 0;display:none}
.quiz-q.answered .why{display:block}
.qa{border-bottom:1px solid var(--line);padding:10px 2px}
.qa .q{font-weight:600;cursor:pointer;list-style:none;display:flex;gap:8px}
.qa .q::-webkit-details-marker{display:none}
.qa .q::before{content:"▸";color:var(--amber);font-weight:700}
.qa[open] .q::before{content:"▾"}
.qa .a{margin:8px 0 4px 18px;color:var(--muted)}
.qa .tag{font-size:11px;font-weight:700;border-radius:4px;padding:1px 7px;margin-left:8px;vertical-align:middle}
.tag.q{background:var(--teal-soft);color:var(--teal-dark)}
.tag.d{background:var(--amber-soft);color:var(--amber)}
.check{list-style:none;padding:0}
.check li{padding:7px 4px;border-bottom:1px dashed var(--line);font-size:15px;display:flex;gap:10px;align-items:flex-start}
.check input{margin-top:5px;accent-color:var(--teal);width:16px;height:16px}
/* home page */
.hero{border:1px solid var(--line);border-radius:14px;padding:26px 28px;background:var(--panel);margin-bottom:22px}
.hero h1{margin:0 0 6px;font-size:32px}
.hero p{color:var(--muted);margin:6px 0}
.path{display:grid;grid-template-columns:repeat(auto-fill,minmax(250px,1fr));gap:12px;margin-top:14px}
.path-card{background:var(--card);border:1px solid var(--line);border-radius:var(--radius);padding:14px 16px;text-decoration:none;color:inherit;position:relative}
.path-card:hover{border-color:var(--teal)}
.path-card .tno{font-family:var(--mono);font-size:12px;color:var(--teal-dark);font-weight:700}
.path-card h3{margin:4px 0;font-size:17px}
.path-card p{margin:4px 0 8px;font-size:13.5px;color:var(--muted)}
.track-title{font-family:var(--sans);font-size:15px;font-weight:700;color:var(--teal-dark);margin:22px 0 4px;text-transform:uppercase;letter-spacing:.05em}
/* overlay (interview drill) */
.overlay{position:fixed;inset:0;background:rgba(28,25,23,.55);display:flex;align-items:center;justify-content:center;z-index:50;padding:20px}
.overlay.hidden{display:none}
.overlay-card{background:#fff;border-radius:14px;max-width:640px;width:100%;max-height:88vh;overflow:auto;padding:26px 28px}
.flash-q{font-family:var(--serif);font-size:22px;margin:10px 0 16px}
.flash-a{background:var(--panel);border:1px solid var(--line);border-radius:10px;padding:12px 16px;font-size:15px}
.flash-meta{font-size:13px;color:var(--faint)}
.pager{display:flex;justify-content:space-between;gap:10px;margin-top:34px;padding-top:16px;border-top:1px solid var(--line)}
.pager a{text-decoration:none;font-weight:600}
.markdone{margin-top:26px;text-align:center}
.search-results{padding:8px 0}
.search-results a{display:block;padding:8px 10px;border-bottom:1px solid var(--line);text-decoration:none;color:inherit}
.search-results a:hover{background:var(--panel)}
.search-results small{color:var(--faint)}
@media(max-width:900px){.sidebar{display:none}body.nav-open .sidebar{display:block;position:fixed;z-index:30;top:0;bottom:0;left:0;max-height:none;box-shadow:2px 0 14px rgba(28,25,23,.25)}.content{padding:20px 16px 80px}.topic-title{font-size:30px}.topbar{flex-wrap:wrap}.topbar-center{order:3;flex-basis:100%}}
@media(prefers-reduced-motion:reduce){html{scroll-behavior:auto}*{animation:none!important;transition:none!important}}
/* ---- robot arm lab ---- */
.arm-stage{display:flex;gap:16px;align-items:flex-start}
#armCanvas{flex:1 1 60%;min-width:0;border:1px solid var(--line);border-radius:10px;background:#fbfdfc;cursor:grab}
.arm-side{flex:1 1 220px;min-width:230px}
.seg{display:flex;flex-wrap:wrap;gap:8px;margin:8px 0 12px}
.seg button{border:1px solid var(--line);background:#fff;border-radius:8px;padding:7px 13px;font-size:13.5px;font-weight:600;cursor:pointer;color:var(--muted)}
.seg button.on{background:var(--teal);border-color:var(--teal);color:#fff}
.seg.amber button.on{background:var(--amber);border-color:var(--amber)}
.arm-banner{display:block;margin-top:16px;padding:14px 18px;background:var(--teal-soft);border:1px solid #c4d9d3;border-radius:12px;text-decoration:none;color:inherit;font-size:15px}
.arm-banner:hover{border-color:var(--teal)}
@media(max-width:760px){.arm-stage{flex-direction:column}#armCanvas{width:100%}}
.plot-full{margin-top:14px;border:1px solid var(--line);border-radius:10px;padding:10px 12px;background:#fff}
.plot-grid{display:grid;grid-template-columns:1fr 1fr;gap:12px;margin-top:12px}
.plot-card{border:1px solid var(--line);border-radius:10px;padding:10px 12px;background:#fff;min-width:0}
.plot-full b,.plot-card b{font-size:13.5px}
@media(max-width:760px){.plot-grid{grid-template-columns:1fr}}