:root {
  --bg:#080b10;--bg2:#0d1117;--bg3:#131920;
  --topbar-bg:rgba(8,11,16,0.88);--glass:rgba(255,255,255,0.04);
  --border:rgba(255,255,255,0.08);--border2:rgba(255,255,255,0.14);
  --shadow:0 2px 16px rgba(0,0,0,0.4);--ink:#f0ede8;--ink2:#9ba4b0;--ink3:#5a6474;
  --gold:#e8c547;--gold-bg:rgba(232,197,71,0.10);--plasma:#4af0c4;
  --plasma-bg:rgba(74,240,196,0.09);--shock:#ff6b6b;--shock-bg:rgba(255,107,107,0.08);
  --violet:#a78bfa;--code-bg:rgba(255,255,255,0.07);--code-col:#4af0c4;
  --r:14px;--r2:20px;--r3:28px;
}
[data-theme="light"] {
  --bg:#f5f2eb;--bg2:#ffffff;--bg3:#f0ece3;--topbar-bg:rgba(245,242,235,0.92);
  --glass:rgba(0,0,0,0.03);--border:rgba(0,0,0,0.08);--border2:rgba(0,0,0,0.13);
  --shadow:0 2px 12px rgba(0,0,0,0.07);--ink:#1a1208;--ink2:#4a4030;--ink3:#9a8e7e;
  --gold:#b8860b;--gold-bg:rgba(184,134,11,0.10);--plasma:#0a7a5a;
  --plasma-bg:rgba(10,122,90,0.09);--shock:#c0392b;--shock-bg:rgba(192,57,43,0.08);
  --violet:#5b35b8;--code-bg:rgba(0,0,0,0.06);--code-col:#0a7a5a;
}
*,*::before,*::after{box-sizing:border-box;margin:0;padding:0;}
html{height:100%;}
body{font-family:'DM Sans',sans-serif;font-size:17px;background:var(--bg);color:var(--ink);line-height:1.6;height:100%;overflow:hidden;}
#bg-canvas{position:fixed;inset:0;z-index:0;pointer-events:none;opacity:0.45;}
#shell{position:relative;z-index:1;display:grid;grid-template-rows:68px 1fr;grid-template-columns:320px 1fr;height:100vh;}
#topbar{grid-column:1/-1;display:flex;align-items:center;gap:20px;padding:0 28px;background:var(--topbar-bg);backdrop-filter:blur(20px);border-bottom:1px solid var(--border);position:relative;z-index:10;}
.logo{font-family:'DM Serif Display',serif;font-size:24px;letter-spacing:-0.5px;color:var(--ink);flex-shrink:0;}
.logo em{font-style:italic;color:var(--gold);}
.logo-sep{width:1px;height:28px;background:var(--border2);}
.tagline{font-family:'Space Mono',monospace;font-size:11px;color:var(--ink3);letter-spacing:1.5px;text-transform:uppercase;flex:1;}
.top-controls{display:flex;align-items:center;gap:12px;}
.filter-group{display:flex;gap:8px;}
.fpill{font-family:'DM Sans',sans-serif;font-size:13px;font-weight:500;padding:7px 18px;border-radius:100px;border:1px solid var(--border2);background:transparent;color:var(--ink3);cursor:pointer;transition:all .2s;}
.fpill:hover{border-color:var(--gold);color:var(--gold);}
.fpill.on{background:var(--gold);border-color:var(--gold);color:#000;}
#theme-toggle{width:38px;height:38px;border-radius:50%;border:1px solid var(--border2);background:var(--glass);color:var(--ink2);cursor:pointer;display:flex;align-items:center;justify-content:center;font-size:18px;flex-shrink:0;transition:all .2s;}
#theme-toggle:hover{border-color:var(--gold);color:var(--gold);}
#sidebar{grid-column:1;grid-row:2;overflow-y:auto;border-right:1px solid var(--border);background:var(--bg2);scrollbar-width:thin;scrollbar-color:var(--border2) transparent;}
.sidebar-header{padding:18px 22px 14px;font-family:'Space Mono',monospace;font-size:10px;letter-spacing:2.5px;text-transform:uppercase;color:var(--ink3);border-bottom:1px solid var(--border);position:sticky;top:0;background:var(--bg2);z-index:2;}
.sitem{padding:16px 22px;cursor:pointer;position:relative;border-bottom:1px solid rgba(255,255,255,0.03);transition:background .15s;overflow:hidden;}
.sitem::before{content:'';position:absolute;left:0;top:0;bottom:0;width:0;transition:width .25s;}
.sitem:hover{background:rgba(255,255,255,0.03);}
.sitem.active{background:rgba(255,255,255,0.05);}
.sitem.active::before{width:3px;}
.sitem-rank{font-family:'Space Mono',monospace;font-size:10px;letter-spacing:2px;color:var(--ink3);margin-bottom:5px;}
.sitem-name{font-size:15px;font-weight:600;color:var(--ink);margin-bottom:8px;line-height:1.3;}
.sitem-foot{display:flex;align-items:center;gap:8px;}
.slevel{font-size:10px;font-weight:600;padding:2px 8px;border-radius:100px;}
.slevel-hs{background:rgba(232,197,71,0.12);color:var(--gold);}
.slevel-uni{background:rgba(167,139,250,0.12);color:var(--violet);}
.slevel-bridge{background:rgba(74,240,196,0.12);color:var(--plasma);}
.sbar{flex:1;height:2px;background:rgba(255,255,255,0.07);border-radius:2px;overflow:hidden;}
.sbar-fill{height:100%;border-radius:2px;transition:width .4s ease;}
.sitem.filtered-out{display:none;}
#stage{grid-column:2;grid-row:2;display:flex;flex-direction:column;position:relative;background:var(--bg);overflow:hidden;}
#welcome{position:absolute;inset:0;display:flex;align-items:center;justify-content:center;padding:60px 40px;transition:opacity .4s,transform .4s;z-index:5;overflow-y:auto;}
#welcome.gone{opacity:0;transform:scale(0.96);pointer-events:none;}
.welcome-inner{max-width:680px;}
.welcome-kicker{font-family:'Space Mono',monospace;font-size:11px;letter-spacing:3px;text-transform:uppercase;color:var(--plasma);margin-bottom:20px;}
.welcome-title{font-family:'DM Serif Display',serif;font-size:62px;line-height:1.05;letter-spacing:-2px;color:var(--ink);margin-bottom:24px;}
.welcome-title em{font-style:italic;color:var(--gold);}
.welcome-body{font-size:19px;color:var(--ink2);line-height:1.8;margin-bottom:40px;max-width:520px;}
.welcome-grid{display:grid;grid-template-columns:repeat(3,1fr);gap:16px;margin-bottom:40px;}
.wcard{background:var(--glass);border:1px solid var(--border);border-radius:var(--r2);padding:24px 20px;transition:border-color .2s,transform .2s;}
.wcard:hover{border-color:var(--border2);transform:translateY(-2px);}
.wcard-icon{font-size:32px;margin-bottom:12px;}
.wcard-title{font-size:14px;font-weight:700;letter-spacing:.5px;text-transform:uppercase;color:var(--ink3);margin-bottom:8px;}
.wcard-desc{font-size:14px;color:var(--ink2);line-height:1.6;}
.welcome-cta{display:inline-flex;align-items:center;gap:8px;font-size:15px;font-weight:600;padding:14px 32px;border-radius:100px;background:var(--gold);color:#000;border:none;cursor:pointer;transition:transform .2s,box-shadow .2s;box-shadow:0 0 40px rgba(232,197,71,0.25);}
.welcome-cta:hover{transform:translateY(-2px);box-shadow:0 4px 50px rgba(232,197,71,0.4);}
#topic-panel{display:flex;flex-direction:column;flex:1;opacity:0;transform:translateX(30px);pointer-events:none;transition:opacity .35s cubic-bezier(.25,1,.5,1),transform .35s cubic-bezier(.25,1,.5,1);min-height:0;}
#topic-panel.visible{opacity:1;transform:none;pointer-events:all;}
#tab-bar{display:flex;align-items:stretch;background:var(--bg2);border-bottom:1px solid var(--border);padding:0 32px;flex-shrink:0;gap:4px;}
.tbutton{font-family:'DM Sans',sans-serif;font-size:14px;font-weight:600;padding:16px 24px 14px;border:none;background:transparent;color:var(--ink3);cursor:pointer;border-bottom:2px solid transparent;margin-bottom:-1px;transition:all .15s;}
.tbutton:hover{color:var(--ink);}
.tbutton.on{color:var(--ink);border-bottom-color:var(--gold);}
.tpanel{flex:1;overflow-y:auto;display:none;scrollbar-width:thin;scrollbar-color:var(--border2) transparent;padding-bottom:60px;}
.tpanel.on{display:block;}
.explain-wrap{padding:48px 56px;max-width:960px;margin:0 auto;}
.topic-eyebrow{font-family:'Space Mono',monospace;font-size:11px;letter-spacing:2.5px;text-transform:uppercase;color:var(--ink3);margin-bottom:16px;display:flex;align-items:center;gap:12px;}
.topic-eyebrow::after{content:'';flex:1;height:1px;background:var(--border);max-width:200px;}
.topic-name{font-family:'DM Serif Display',serif;font-size:52px;letter-spacing:-1.5px;line-height:1.05;margin-bottom:20px;}
.topic-hook{font-size:19px;color:var(--ink2);line-height:1.8;max-width:720px;margin-bottom:44px;}
.core-idea{border-radius:var(--r3);padding:36px 40px;margin-bottom:48px;position:relative;overflow:hidden;}
.core-idea::before{content:'';position:absolute;inset:0;background:currentColor;opacity:0.07;}
.core-idea-label{font-family:'Space Mono',monospace;font-size:10px;letter-spacing:3px;text-transform:uppercase;opacity:0.6;margin-bottom:12px;position:relative;z-index:1;}
.core-idea-text{font-family:'DM Serif Display',serif;font-size:26px;line-height:1.55;letter-spacing:-0.3px;position:relative;z-index:1;}
.analogy-grid{display:grid;grid-template-columns:1fr 1fr;gap:20px;margin-bottom:48px;}
.abox{border-radius:var(--r2);padding:28px 30px;border:1px solid var(--border);}
.abox.everyday{background:rgba(232,197,71,0.05);border-color:rgba(232,197,71,0.2);}
.abox.physics{background:rgba(74,240,196,0.05);border-color:rgba(74,240,196,0.2);}
.abox-label{font-family:'Space Mono',monospace;font-size:10px;letter-spacing:2px;text-transform:uppercase;margin-bottom:12px;}
.abox.everyday .abox-label{color:var(--gold);}
.abox.physics .abox-label{color:var(--plasma);}
.abox-text{font-size:16px;color:var(--ink2);line-height:1.8;}
.steps-label{font-family:'Space Mono',monospace;font-size:10px;letter-spacing:2.5px;text-transform:uppercase;color:var(--ink3);margin-bottom:24px;display:flex;align-items:center;gap:10px;}
.steps-label::after{content:'';flex:1;height:1px;background:var(--border);max-width:300px;}
.step{display:flex;gap:32px;padding:28px 0;border-bottom:1px solid var(--border);opacity:0;transform:translateY(16px);transition:opacity .4s ease,transform .4s ease;}
.step:last-child{border-bottom:none;}
.step.in{opacity:1;transform:none;}
.step-num{font-family:'DM Serif Display',serif;font-style:italic;font-size:64px;color:rgba(255,255,255,0.06);line-height:1;width:70px;flex-shrink:0;text-align:right;padding-top:4px;}
.step-content{flex:1;}
.step-title{font-size:18px;font-weight:700;margin-bottom:10px;color:var(--ink);}
.step-body{font-size:16px;color:var(--ink2);line-height:1.8;}
.step-body code{font-family:'Space Mono',monospace;font-size:13px;background:var(--code-bg);padding:2px 8px;border-radius:5px;color:var(--plasma);}
.interactive-zone{background:var(--bg3);border:1px solid var(--border2);border-radius:var(--r3);padding:32px 36px;margin:32px 0;}
.iz-label{font-family:'Space Mono',monospace;font-size:11px;letter-spacing:2.5px;text-transform:uppercase;color:var(--plasma);margin-bottom:16px;display:flex;align-items:center;gap:10px;}
.slider-row{display:flex;align-items:center;gap:20px;margin-bottom:16px;flex-wrap:wrap;}
.slider-row label{font-size:14px;font-weight:500;color:var(--ink2);min-width:180px;}
.slider-row input[type=range]{flex:1;min-width:180px;}
.slider-row input[type=number]{flex:1;min-width:120px;background:var(--bg2);border:1px solid var(--border);padding:10px;border-radius:8px;color:var(--ink);}
.slider-val{font-family:'Space Mono',monospace;font-size:14px;color:var(--gold);min-width:90px;}
.iz-output{font-family:'Space Mono',monospace;font-size:18px;color:var(--plasma);padding:20px 24px;background:var(--bg2);border-radius:var(--r);margin-top:16px;border-left:3px solid var(--gold);}
.iz-output span{color:var(--gold);font-size:24px;font-weight:700;}
.myths-wrap{margin-top:16px;}
.myth{background:rgba(255,107,107,0.06);border:1px solid rgba(255,107,107,0.2);border-left:4px solid var(--shock);border-radius:var(--r2);padding:20px 26px;margin-bottom:14px;}
.myth-wrong{font-size:15px;font-weight:700;color:var(--shock);margin-bottom:10px;}
.myth-wrong::before{content:'✗  ';font-size:18px;}
.myth-right{font-size:15px;color:var(--ink2);line-height:1.75;}
.myth-right::before{content:'✓  ';color:var(--plasma);font-weight:700;font-size:18px;}
.quiz-wrap{padding:48px 56px;max-width:820px;margin:0 auto;}
.quiz-header{margin-bottom:32px;}
.quiz-topic-name{font-family:'DM Serif Display',serif;font-size:34px;letter-spacing:-.5px;color:var(--ink2);margin-bottom:8px;}
.quiz-progress-bar{height:4px;background:var(--border);border-radius:4px;overflow:hidden;margin-bottom:8px;}
.quiz-progress-fill{height:100%;border-radius:4px;transition:width .4s ease;}
.quiz-counter{font-family:'Space Mono',monospace;font-size:13px;color:var(--ink3);letter-spacing:1px;}
.qcard{background:var(--bg3);border:1px solid var(--border);border-radius:var(--r3);padding:40px 44px;margin-bottom:24px;}
.qdiff{font-family:'Space Mono',monospace;font-size:11px;letter-spacing:2px;text-transform:uppercase;margin-bottom:18px;}
.qtext{font-family:'DM Serif Display',serif;font-size:28px;letter-spacing:-.4px;line-height:1.4;color:var(--ink);margin-bottom:32px;}
.qopts{display:flex;flex-direction:column;gap:12px;}
.qopt{font-family:'DM Sans',sans-serif;font-size:16px;text-align:left;background:var(--bg2);border:1.5px solid var(--border);border-radius:var(--r2);padding:16px 24px;cursor:pointer;color:var(--ink2);transition:all .2s;display:flex;align-items:center;gap:16px;}
.qopt:hover:not(:disabled){border-color:var(--gold);color:var(--gold);background:rgba(232,197,71,0.06);}
.qopt-letter{font-family:'Space Mono',monospace;font-size:13px;font-weight:700;width:32px;height:32px;border-radius:50%;display:flex;align-items:center;justify-content:center;background:var(--border);flex-shrink:0;}
.qopt.correct{border-color:var(--plasma);color:var(--plasma);background:rgba(74,240,196,0.07);}
.qopt.correct .qopt-letter{background:var(--plasma);color:#000;}
.qopt.wrong{border-color:var(--shock);color:var(--shock);background:rgba(255,107,107,0.07);}
.qopt.wrong .qopt-letter{background:var(--shock);color:#000;}
.qopt:disabled{cursor:default;}
.qexplain{border-radius:var(--r2);padding:24px 28px;font-size:16px;line-height:1.8;color:var(--ink2);margin-top:8px;display:none;border-left:4px solid var(--plasma);background:rgba(74,240,196,0.06);}
.qexplain.show{display:block;}
.qexplain.wrong-exp{border-left-color:var(--shock);background:rgba(255,107,107,0.06);}
.qexplain-label{font-family:'Space Mono',monospace;font-size:11px;letter-spacing:2px;text-transform:uppercase;color:var(--plasma);margin-bottom:10px;}
.qnext{font-family:'DM Sans',sans-serif;font-size:16px;font-weight:600;padding:14px 36px;border-radius:100px;border:none;background:var(--gold);color:#000;cursor:pointer;display:none;margin-top:20px;transition:transform .2s;}
.qnext:hover{transform:translateY(-1px);}
.qnext.show{display:inline-block;}
.score-box{display:none;padding:80px 0;text-align:center;}
.score-box.show{display:block;}
.score-ring{width:160px;height:160px;border-radius:50%;border:4px solid var(--border);display:flex;align-items:center;justify-content:center;flex-direction:column;margin:0 auto 32px;position:relative;}
.score-ring::before{content:'';position:absolute;inset:-4px;border-radius:50%;border:4px solid transparent;border-top-color:var(--gold);animation:spin 4s linear infinite;}
@keyframes spin{to{transform:rotate(360deg);}}
.score-num{font-family:'DM Serif Display',serif;font-size:68px;letter-spacing:-3px;line-height:1;color:var(--ink);}
.score-denom{font-size:18px;color:var(--ink3);}
.score-verdict{font-family:'DM Serif Display',serif;font-size:36px;letter-spacing:-.5px;margin-bottom:16px;}
.score-note{font-size:17px;color:var(--ink2);line-height:1.7;max-width:500px;margin:0 auto 36px;}
.score-actions{display:flex;gap:16px;justify-content:center;}
.btn-ghost{font-size:15px;font-weight:600;padding:14px 32px;border-radius:100px;border:1.5px solid var(--border2);background:transparent;color:var(--ink);cursor:pointer;transition:all .2s;}
.btn-ghost:hover{border-color:var(--gold);color:var(--gold);}
.btn-solid{font-size:15px;font-weight:600;padding:14px 32px;border-radius:100px;border:none;background:var(--gold);color:#000;cursor:pointer;transition:transform .2s;}
.btn-solid:hover{transform:translateY(-1px);}
#upgrade-banner{position:fixed;bottom:0;left:0;right:0;z-index:200;background:linear-gradient(135deg,rgba(8,11,16,0.98),rgba(13,17,23,0.99));border-top:1px solid var(--border2);padding:18px 36px;display:flex;align-items:center;gap:24px;transform:translateY(100%);transition:transform .4s cubic-bezier(.25,1,.5,1);backdrop-filter:blur(20px);}
#upgrade-banner.show{transform:none;}
.ub-text{flex:1;font-size:15px;color:var(--ink2);line-height:1.6;}
.ub-text strong{color:var(--gold);}
.ub-price{font-family:'DM Serif Display',serif;font-size:24px;color:var(--ink);}
.ub-cta{font-size:14px;font-weight:700;padding:12px 28px;border-radius:100px;background:var(--gold);color:#000;border:none;cursor:pointer;transition:transform .2s;}
.ub-cta:hover{transform:translateY(-1px);}
.ub-dismiss{font-size:24px;color:var(--ink3);background:none;border:none;cursor:pointer;padding:4px 8px;}
#site-footer{text-align:center;padding:14px;background:var(--bg2);border-top:1px solid var(--border);font-size:12px;color:var(--ink2);flex-shrink:0;position:relative;z-index:5;}
@media(max-width:780px){
  #shell{grid-template-columns:1fr;grid-template-rows:68px 1fr;height:100vh;}
  #sidebar{position:fixed;left:-300px;top:68px;width:300px;height:calc(100% - 68px);z-index:100;transition:left 0.25s ease;background:var(--bg2);overflow-y:auto;}
  #sidebar.open{left:0;}
  #stage{grid-column:1;width:100%;}
  #topbar{position:sticky;top:0;z-index:50;}
  .filter-group{display:none;}
  .tagline{display:none;}
  .top-controls{margin-left:auto;}
  .explain-wrap,.quiz-wrap{padding:24px 16px;width:100%;}
  .topic-name{font-size:32px;word-break:break-word;}
  .topic-hook{font-size:17px;}
  .core-idea-text{font-size:20px;}
  .analogy-grid{grid-template-columns:1fr;}
  .step{flex-direction:column;gap:12px;}
  .step-num{width:100%;text-align:left;font-size:48px;}
  .slider-row{flex-direction:column;align-items:flex-start;}
  .slider-row label{min-width:auto;}
  .slider-row input[type=range]{width:100%;}
  .qcard{padding:24px;}
  .qtext{font-size:22px;}
  .qopt{font-size:15px;padding:14px 18px;}
  .welcome-title{font-size:36px;}
  .welcome-grid{grid-template-columns:1fr;}
  .score-num{font-size:56px;}
  .score-verdict{font-size:28px;}
  .interactive-zone{padding:20px;}
}
::-webkit-scrollbar{width:6px;}
::-webkit-scrollbar-track{background:transparent;}
::-webkit-scrollbar-thumb{background:var(--border2);border-radius:10px;}