:root{color-scheme:light;--paper:#faf9f6;--ink:#202d35;--muted:#626d77;--line:#e1dfd8;--surface:#fffdfb;--accent:#4677ac;--tint:#eaf1f8;--serif:Georgia,"Songti SC","STSong",serif;--sans:-apple-system,BlinkMacSystemFont,"PingFang SC","Helvetica Neue",sans-serif;font-family:var(--sans);background:var(--paper);color:var(--ink)}
*{box-sizing:border-box}body{margin:0}a{color:inherit;text-decoration:none}button,input,textarea,select{font:inherit;color:inherit}button,a,input,select{touch-action:manipulation}button{cursor:pointer}button:disabled{cursor:default;opacity:.45}button:focus-visible,a:focus-visible,summary:focus-visible,input:focus-visible,textarea:focus-visible{outline:3px solid #b78b3e;outline-offset:4px}button{background:transparent;border:0}svg{display:block}button svg{pointer-events:none}h1,h2,h3,p{margin:0}h1,h2,h3{font-weight:500}img{display:block;max-width:100%}.skip{position:fixed;left:16px;top:-80px;z-index:100;background:#fff;padding:12px}.skip:focus{top:10px}
.site-header{max-width:1440px;padding:25px 40px;margin:auto;display:flex;gap:32px;align-items:center;justify-content:space-between;border-bottom:1px solid var(--line)}.brand{display:flex;align-items:center;gap:13px;font-family:var(--serif);font-size:27px;white-space:nowrap}.brand img{width:38px;height:38px}.brand small{font-size:12px;letter-spacing:.1em;display:block;color:var(--muted);margin-top:3px;font-family:var(--sans)}.main-nav{display:flex;gap:25px;align-items:center}.main-nav a,.account-button{font-size:14px;min-height:44px;display:flex;align-items:center;gap:7px}.main-nav a[aria-current=page]{color:var(--accent);border-bottom:2px solid var(--accent)}.account-button{border-left:1px solid var(--line);padding-left:22px;max-width:180px;overflow:hidden;text-overflow:ellipsis}.icon{width:24px;height:24px;flex-shrink:0}.small-icon{width:18px;height:18px}.account-label{overflow:hidden;text-overflow:ellipsis;white-space:nowrap}#main{max-width:1360px;min-height:65vh;margin:0 auto;padding:32px 0 64px}.loading,.empty{padding:72px 20px;color:var(--muted);text-align:center}.error{padding:24px;background:#fff0e9;color:#8b4026;border-radius:12px;line-height:1.7}
.home-tools{display:flex;justify-content:space-between;align-items:center;margin:0 0 28px;gap:20px}.eyebrow{font-size:12px;letter-spacing:.12em;color:var(--muted)}.home-count{font-family:var(--serif);font-size:22px;margin-top:6px}.search-wrap{display:flex;align-items:center;gap:9px;border-bottom:1px solid #c7c6c0;padding:7px 2px;width:260px}.search-wrap input{background:none;border:0;outline:0;width:100%;font-size:14px;min-height:32px}.themes{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:28px}.theme-card{background:var(--surface);border:1px solid #e8e5df;border-radius:18px;overflow:hidden;transition:transform .2s,box-shadow .2s}.theme-card:hover{transform:translateY(-3px);box-shadow:0 12px 35px #3d34200d}.theme-art{aspect-ratio:2/1;background:var(--tint);overflow:hidden}.theme-art img{width:100%;height:100%;object-fit:cover}.theme-meta{padding:21px 25px;display:flex;align-items:center;justify-content:space-between;gap:20px}.theme-title{display:flex;gap:12px;align-items:center}.theme-title .icon{color:var(--accent)}.theme-title h2{font-family:var(--serif);font-size:26px;line-height:1.3}.theme-title p{font-size:13px;color:var(--muted);margin-top:5px}.theme-count{font-size:13px;color:var(--muted);white-space:nowrap}.theme-count span{display:block;text-align:right;margin-top:7px;color:var(--accent)}.resume{margin:0 0 28px;background:var(--surface);border-left:3px solid #a68550;border-radius:5px;display:flex;align-items:center;justify-content:space-between;padding:16px 20px;gap:20px;font-size:14px}.resume span{color:var(--muted);margin-left:8px}.text-link{color:var(--accent);font-size:14px;display:inline-flex;align-items:center;min-height:44px;gap:6px}.quiet-note{font-size:12px;color:var(--muted);line-height:1.7;margin-top:24px}.notice{border-left:3px solid #b78043;background:#fff3df;padding:15px 18px;line-height:1.8;font-size:14px;border-radius:5px;margin:16px 0}
.topic-heading{display:grid;grid-template-columns:1fr 250px;align-items:center;gap:25px;margin:8px 0 30px;border-bottom:1px solid var(--line);padding-bottom:28px}.topic-heading h1{font-family:var(--serif);font-size:33px;line-height:1.4;margin:12px 0 8px}.topic-heading p{font-size:14px;color:var(--muted)}.topic-heading img{border-radius:14px;aspect-ratio:2/1;object-fit:cover;width:250px}.crumb{font-size:13px;color:var(--muted);display:flex;gap:10px;align-items:center;flex-wrap:wrap;min-height:30px}.topic-tabs{display:flex;gap:8px;flex-wrap:wrap;margin:24px 0}.topic-tabs a,.pill{min-height:40px;padding:10px 16px;border-radius:24px;font-size:13px;display:inline-flex;align-items:center;gap:6px}.topic-tabs a[aria-current=true],.pill.active{background:var(--accent);color:#fff}.topic-tabs a:not([aria-current=true]){background:#eeede8}.catalog-toolbar{display:flex;justify-content:space-between;align-items:center;gap:15px;margin:22px 0}.filters{display:flex;gap:8px;flex-wrap:wrap}.filters button{border:1px solid var(--line);padding:9px 15px;border-radius:22px;font-size:13px;min-height:40px}.filters button[aria-pressed=true]{background:var(--ink);color:#fff;border-color:var(--ink)}.question-list{display:grid;grid-template-columns:repeat(3,minmax(0,1fr));gap:18px}.question-card{padding:23px;border:1px solid var(--line);border-radius:14px;background:var(--surface);position:relative;min-height:170px;transition:border-color .2s}.question-card:hover{border-color:var(--accent)}.card-top{display:flex;align-items:center;justify-content:space-between;margin-bottom:18px}.q-number{font-family:var(--serif);font-size:20px;color:var(--accent)}.status{font-size:11px;color:var(--muted);background:#f0eee8;padding:4px 8px;border-radius:6px}.status.wrong{background:#fae9e1;color:#a24d2f}.status.correct{background:#e6f0e8;color:#356b4b}.question-card h2{font-family:var(--serif);font-size:22px;line-height:1.4;margin-bottom:6px}.question-card p{font-size:12px;color:var(--muted);line-height:1.5;max-width:90%}.source-code{font-size:11px;color:var(--muted);margin-top:17px;letter-spacing:.02em}.card-arrow{position:absolute;right:20px;bottom:20px;font-size:21px;color:var(--accent)}
.lesson-top{display:flex;justify-content:space-between;gap:20px;align-items:flex-start;margin-bottom:24px}.lesson-top h1{font-family:var(--serif);font-size:31px;margin:12px 0 5px;line-height:1.35}.lesson-subtitle{font-size:13px;color:var(--muted)}.lesson-actions{display:flex;gap:8px;flex-wrap:wrap;justify-content:flex-end}.soft-btn{border:1px solid var(--line);background:var(--surface);border-radius:24px;padding:10px 16px;min-height:44px;font-size:13px;display:inline-flex;align-items:center;justify-content:center;gap:7px}.soft-btn.active{background:var(--tint);border-color:var(--accent);color:var(--accent)}.primary{background:var(--accent);border:1px solid var(--accent);color:white;border-radius:9px;padding:12px 22px;min-height:46px;font-size:14px}.secondary{border:1px solid var(--line);background:var(--surface);border-radius:9px;padding:12px 20px;min-height:46px;font-size:14px}.lesson-grid{display:grid;grid-template-columns:minmax(0,1fr) minmax(0,1.15fr);gap:30px;align-items:start}.question-pane,.explanation-pane{min-width:0}.pane-title{font-size:12px;color:var(--accent);letter-spacing:.12em;margin:0 0 17px;display:flex;justify-content:space-between;gap:10px;align-items:center}.question-body{background:var(--surface);border:1px solid var(--line);border-radius:15px;padding:27px;font-family:Georgia,"Times New Roman",serif;font-size:17px;line-height:1.9;overflow-wrap:break-word}.question-body p{margin:0 0 12px}.question-body table{border-collapse:collapse;width:100%;font-size:14px;margin:10px 0}.question-body td,.question-body th{border:1px solid #d7d5d0;padding:6px;white-space:normal}.question-body .katex{font-size:1.08em}.math{display:inline}.source-image{margin-top:17px;border:1px solid var(--line);border-radius:12px;overflow:hidden;background:#fff}.source-image summary{padding:13px 16px;font-size:13px;cursor:pointer;min-height:44px}.source-image button{display:block;width:100%;padding:12px;background:#fff}.source-image img{max-height:360px;max-width:100%;margin:auto;object-fit:contain}.source-image figcaption{text-align:center;font-size:12px;color:var(--muted);padding:0 10px 14px}.answer-box{border-top:1px solid var(--line);padding-top:22px;margin-top:22px}.answer-label{font-size:13px;color:var(--muted);margin-bottom:12px}.answer-options{display:flex;gap:10px}.option{width:50px;min-height:46px;border:1px solid #c9cccd;border-radius:9px;background:white;display:flex;align-items:center;justify-content:center;font-family:Georgia,serif;font-size:17px}.option[aria-pressed=true]{background:var(--accent);border-color:var(--accent);color:#fff}.numeric{border:1px solid #bfc5c8;background:white;padding:12px 14px;border-radius:9px;width:100%;min-height:46px;max-width:330px}.answer-buttons{display:flex;gap:12px;flex-wrap:wrap;margin-top:18px}.feedback{font-size:14px;line-height:1.7;min-height:22px;margin-top:15px}.feedback.bad{color:#99492c}.feedback.good{color:#28714e}.source-full{font-size:13px;color:var(--muted);line-height:1.8;margin:20px 0}.source-full summary{cursor:pointer}.source-full div{padding-top:12px}.source-full p{margin:8px 0}
.step-intro{border:1px solid var(--line);border-radius:16px;background:var(--surface);overflow:hidden}.step-intro-art{height:240px;background:var(--tint)}.step-intro-art img{width:100%;height:100%;object-fit:cover}.step-intro-body{padding:24px}.step-intro h2{font-family:var(--serif);font-size:25px;margin-bottom:10px}.step-intro p{font-size:14px;color:var(--muted);line-height:1.8;margin-bottom:20px}.step-controls{display:flex;gap:10px;justify-content:space-between;align-items:center;margin-bottom:18px;flex-wrap:wrap}.step-dots{display:flex;gap:8px}.step-dot{width:35px;height:35px;border:1px solid var(--line);border-radius:50%;font-size:13px}.step-dot[aria-current=step]{background:var(--accent);color:#fff;border-color:var(--accent)}.teaching-stage{border:1px solid var(--line);border-radius:18px;background:linear-gradient(155deg,var(--tint),#fffdf9);overflow:hidden;position:relative}.stage-header{display:flex;justify-content:space-between;padding:20px 23px 0;font-size:11px;color:var(--accent);letter-spacing:.1em}.visual{padding:10px 28px 0;min-height:200px;display:flex;align-items:center;justify-content:center}.visual svg{width:100%;max-height:270px;overflow:visible}.visual text{font:13px var(--sans);fill:#394950}.visual .soft{fill:var(--tint);stroke:var(--accent)}.visual-caption{font-size:11px;text-align:center;color:var(--muted);margin-bottom:8px}.formula-stage{padding:24px 12px 32px;text-align:center;overflow-x:auto;overscroll-behavior-x:contain}.formula-stage .katex{font-size:1.65em}.formula-history{display:flex;justify-content:center;gap:10px;padding:0 18px 20px;flex-wrap:wrap;font-size:13px;color:var(--muted)}.formula-history .math{max-width:100%;overflow:auto;background:#ffffffb0;border-radius:5px;padding:5px 8px}.explainer-text{padding:22px 3px}.explainer-text .zh{font-size:17px;line-height:1.9}.explainer-text .en{font-family:Georgia,serif;font-size:15px;line-height:1.8;color:var(--muted);margin-top:12px}.step-footer{display:flex;justify-content:space-between;align-items:center;gap:10px;border-top:1px solid var(--line);padding-top:18px;margin-top:10px}.stage-range{padding:0 24px 17px;display:flex;gap:14px;align-items:center;font-size:13px}.stage-range input{min-width:0;width:100%;accent-color:var(--accent)}.lesson-bottom{display:flex;justify-content:space-between;align-items:center;border-top:1px solid var(--line);margin-top:40px;padding-top:20px;gap:16px;font-size:14px}.lesson-bottom a{max-width:45%;line-height:1.6}.step-fade{animation:step-in .25s ease-out}@keyframes step-in{from{opacity:.5;transform:translateY(5px)}to{opacity:1;transform:none}}
.review-heading{font-family:var(--serif);font-size:32px;margin:9px 0 12px}.stats{display:flex;gap:24px;flex-wrap:wrap;margin:25px 0 30px;padding:20px 0;border-top:1px solid var(--line);border-bottom:1px solid var(--line)}.stat strong{font-family:var(--serif);font-size:31px;font-weight:400;margin-right:8px}.stat span{font-size:13px;color:var(--muted)}.test-intro{max-width:850px;margin:18px auto 20px}.test-intro h1{font-family:var(--serif);font-size:36px;margin:10px 0 18px}.test-intro>p{font-size:16px;color:var(--muted);line-height:1.9}.test-themes{display:grid;grid-template-columns:repeat(4,minmax(0,1fr));gap:12px;margin:30px 0}.test-theme{background:var(--surface);border:1px solid var(--line);border-radius:12px;padding:15px;font-size:13px;line-height:1.6}.test-theme svg{color:var(--accent);margin-bottom:10px}.test-theme small{color:var(--muted)}.test-head{display:flex;align-items:center;justify-content:space-between;margin-bottom:20px;gap:15px}.test-layout{max-width:820px;margin:auto}.test-progress{display:flex;gap:8px;flex-wrap:wrap;margin-bottom:22px}.test-progress button{width:36px;height:36px;border:1px solid var(--line);border-radius:8px}.test-progress button.answered{background:var(--tint);border-color:var(--accent)}.test-progress button[aria-current=step]{background:var(--accent);color:#fff}.test-foot{display:flex;justify-content:space-between;gap:10px;flex-wrap:wrap;margin-top:24px}.test-status{font-size:12px;color:var(--muted)}.test-results h1{font-family:var(--serif);font-size:36px;margin:12px 0}.score{font-family:var(--serif);font-size:64px;margin:20px 0}.score small{font-size:26px;color:var(--muted)}.topic-result{padding:17px 0;border-bottom:1px solid var(--line);display:flex;justify-content:space-between;font-size:15px}.result-items{margin-top:32px}.result-row{display:flex;align-items:center;justify-content:space-between;padding:15px 0;border-bottom:1px solid var(--line);gap:12px;font-size:14px}.result-row span{color:var(--muted)}
.site-footer{max-width:1360px;margin:auto;display:flex;justify-content:space-between;gap:16px;border-top:1px solid var(--line);padding:25px 0 35px;font-size:12px;color:var(--muted);flex-wrap:wrap}dialog{border:1px solid var(--line);border-radius:18px;padding:28px;max-width:450px;width:calc(100% - 32px);background:var(--paper);color:var(--ink);box-shadow:0 20px 100px #0003}dialog::backdrop{background:#1e2d3d60;backdrop-filter:blur(3px)}.dialog-head{display:flex;align-items:center;justify-content:space-between;gap:15px;margin-bottom:22px}.dialog-head h2{font-family:var(--serif);font-size:25px}.close-dialog{font-size:26px;min-width:40px;min-height:40px}.account-form{display:grid;gap:16px}.account-form label{font-size:13px;display:grid;gap:9px}.account-form input{padding:13px;border:1px solid #c7cacb;border-radius:8px;background:white;min-width:0}.dialog-copy{color:var(--muted);font-size:13px;line-height:1.8;margin:15px 0}.form-error{color:#99492c;font-size:13px;line-height:1.6;min-height:19px}.dialog-switch{margin:15px 0 0;color:var(--accent);font-size:14px;min-height:40px}#image-dialog{max-width:1100px;width:calc(100% - 24px);max-height:94vh;padding:15px}#image-dialog .dialog-head{position:sticky;top:0;background:var(--paper);padding:5px 0}#image-dialog img{margin:auto;height:auto}.toast{position:fixed}#toast{position:fixed;bottom:28px;left:50%;transform:translateX(-50%);background:#283933;color:#fff;padding:12px 20px;border-radius:10px;font-size:13px;max-width:90%;z-index:200;display:none}#toast.show{display:block}.account-page{max-width:680px;margin:auto}.account-page h1{font-family:var(--serif);font-size:32px;margin:15px 0}.account-page p{color:var(--muted);line-height:1.8;margin:15px 0}.account-page .primary{margin-top:15px}
@media(min-width:1600px){#main,.site-footer{max-width:1360px}}@media(max-width:1450px){#main,.site-footer{margin-left:40px;margin-right:40px}}@media(max-width:1050px){.site-header{padding:22px 28px;gap:20px}.brand{font-size:23px}.main-nav{gap:15px}#main,.site-footer{margin-left:28px;margin-right:28px}.question-list{grid-template-columns:repeat(2,minmax(0,1fr))}.lesson-grid{gap:24px}.question-body{padding:21px;font-size:16px}.formula-stage .katex{font-size:1.35em}.theme-title h2{font-size:23px}.theme-meta{padding:20px}}
@media(max-width:760px){.site-header{flex-wrap:wrap;padding:18px 20px 12px;gap:14px}.brand{font-size:23px}.brand img{width:32px;height:32px}.brand small{font-size:10px}.main-nav{width:100%;justify-content:space-between;gap:10px}.main-nav a,.account-button{font-size:13px;min-height:40px}.account-button{max-width:130px;padding-left:12px}.main-nav a[aria-current=page]{border-bottom-width:1px}#main{margin:0 20px;padding-top:24px}.site-footer{margin:0 20px;line-height:1.6}.site-footer span:nth-child(2){display:none}.home-tools{align-items:flex-start;margin-bottom:22px;gap:12px}.home-count{font-size:20px}.eyebrow{font-size:10px}.search-wrap{width:45%;min-width:130px}.search-wrap input{font-size:12px}.themes{gap:22px;grid-template-columns:1fr}.theme-title h2{font-size:25px}.theme-meta{padding:18px 20px}.theme-title p{font-size:12px}.resume{padding:14px;align-items:flex-start}.resume span{display:block;margin:6px 0 0;font-size:12px}.resume a{white-space:nowrap}.topic-heading{grid-template-columns:1fr 100px;gap:12px;padding-bottom:20px;margin-top:0}.topic-heading h1{font-size:27px}.topic-heading img{width:100px;border-radius:10px;aspect-ratio:1/1}.topic-heading p{font-size:12px;line-height:1.6}.topic-tabs{gap:6px;margin:18px 0}.topic-tabs a{padding:8px 11px;font-size:12px;min-height:37px}.catalog-toolbar{align-items:flex-start;flex-direction:column}.filters{gap:7px}.filters button{padding:8px 12px;font-size:12px}.question-list{gap:12px}.question-card{padding:17px;min-height:175px}.question-card h2{font-size:20px}.question-card p{font-size:11px}.q-number{font-size:18px}.status{font-size:10px;padding:4px 6px}.card-top{margin-bottom:15px}.card-arrow{right:14px;bottom:16px}.source-code{font-size:10px}.lesson-top{flex-direction:column;margin-bottom:22px;gap:18px}.lesson-top h1{font-size:27px}.lesson-actions{justify-content:flex-start;gap:7px}.soft-btn{font-size:12px;padding:10px 13px}.lesson-grid{grid-template-columns:1fr;gap:32px}.question-body{font-size:17px;padding:20px;line-height:1.85}.question-body table{font-size:12px}.question-body td,.question-body th{padding:5px 3px}.pane-title{margin-bottom:12px}.formula-stage .katex{font-size:1.3em}.formula-stage{padding:25px 10px}.visual{padding:10px 20px 0;min-height:180px}.visual svg{max-height:230px}.stage-header{padding:18px 18px 0}.step-intro-art{height:auto;aspect-ratio:2/1}.step-intro-body{padding:21px}.explainer-text .zh{font-size:16px}.explainer-text .en{font-size:14px}.step-footer{flex-wrap:wrap}.step-footer button{padding:11px 17px}.lesson-bottom{font-size:12px}.source-image img{max-height:400px}.test-intro h1{font-size:30px}.test-intro>p{font-size:14px}.test-themes{grid-template-columns:repeat(2,minmax(0,1fr))}.test-head h1{font-size:21px}.test-results h1{font-size:29px}.stats{gap:18px}.stat strong{font-size:27px}.stat span{font-size:11px}.test-foot .primary,.test-foot .secondary{padding:11px 15px}}
@media(max-width:370px){#main,.site-footer{margin-left:16px;margin-right:16px}.site-header{padding-left:16px;padding-right:16px}.brand{font-size:22px}.question-list{grid-template-columns:1fr}.theme-title h2{font-size:22px}.theme-title{gap:8px}.theme-meta{padding:17px 16px}.theme-count{font-size:12px}.question-body{font-size:16px;padding:17px}.main-nav{gap:8px}.account-button{max-width:105px}.main-nav a,.account-button{font-size:12px}.topic-heading{grid-template-columns:1fr 80px}.topic-heading img{width:80px}.topic-heading h1{font-size:24px}.step-controls{gap:8px}.step-dot{width:32px;height:32px}.score{font-size:55px}.home-count{font-size:18px}.search-wrap{min-width:118px}}
@media(prefers-reduced-motion:reduce){*,*:before,*:after{animation:none!important;transition:none!important;scroll-behavior:auto!important}}@media print{.site-header,.site-footer,.lesson-actions,.answer-buttons,.step-footer,.step-controls{display:none}.lesson-grid{display:block}.question-pane,.explanation-pane{break-inside:avoid}}
.relation-cards{width:100%;display:grid;grid-template-columns:minmax(0,1fr) 25px minmax(0,1fr);gap:12px;align-items:center;margin:25px 0;min-height:130px}.relation-cards>div{background:#fffdfcce;border:1px solid color-mix(in srgb,var(--accent) 25%,transparent);border-radius:14px;padding:25px 12px;text-align:center;overflow-x:auto;box-shadow:0 12px 20px #253b3b06}.relation-cards>span{text-align:center;font-size:26px;color:var(--accent)}.relation-cards .katex{font-size:1.25em}.relation-cards.single{grid-template-columns:minmax(0,1fr)}.table-scroll{max-width:100%;overflow-x:auto;margin:15px 0}@media(max-width:760px){.relation-cards{gap:7px}.relation-cards .katex{font-size:1em}.relation-cards>div{padding:20px 8px}.relation-cards.single .katex{font-size:1.15em}}
@keyframes step-in{from{transform:translateY(3px)}to{transform:none}}
.skip{transform:translateY(-200%);top:0}.skip:focus-visible{top:10px;transform:none}.skip:focus:not(:focus-visible){transform:translateY(-200%)}
@media(max-width:760px){.home-tools{display:block}.home-tools .search-wrap{width:100%;margin-top:13px}.home-count{font-size:20px}}
.skip{clip-path:inset(50%);width:1px;height:1px;overflow:hidden;left:0;top:0;padding:0;transform:none}.skip:focus-visible{clip-path:none;width:auto;height:auto;overflow:visible;left:16px;top:10px;padding:12px;transform:none}

.set-picker{display:flex;align-items:center;gap:15px;flex-wrap:wrap;margin:12px 0 24px;color:var(--muted);font-size:13px}.set-picker select{margin-left:8px;padding:9px 28px 9px 12px;border:1px solid var(--line);border-radius:8px;background:var(--surface);max-width:100%}.set-picker span{font-size:12px}.source-set{font-size:12px;color:var(--muted)}

/* Shared lesson order: original question, titled navigation, explanation, canvas. */
.lesson-page{max-width:1040px;margin:auto}.lesson-page .lesson-top{align-items:center}.lesson-page .question-body{border:0;background:none;padding:0;font-size:18px}.lesson-page .question-pane{padding:28px 32px;background:var(--surface);border:1px solid var(--line);border-radius:16px}.lesson-page .source-image{background:transparent;margin-top:18px}.lesson-page .source-image summary{font-size:12px;color:var(--muted)}.lesson-answer{margin-top:22px}.lesson-options{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:10px}.original-option{border:1px solid var(--line);border-radius:10px;background:#fff;min-height:54px;padding:13px 16px;display:flex;align-items:center;gap:14px;text-align:left;min-width:0;transition:border-color .15s,background .15s}.choice-key{color:var(--muted);font:16px Georgia,serif;flex-shrink:0}.choice-content{font:16px/1.65 Georgia,"Times New Roman",serif;min-width:0;overflow-wrap:anywhere}.choice-content .katex{font-size:1.05em}.original-option:hover{border-color:var(--accent);background:var(--tint)}.original-option[aria-pressed=true]{border-color:var(--accent);background:var(--tint)}.original-option.is-correct{border-color:#397453;background:#edf5ee}.original-option.is-wrong{border-color:#b57654;background:#fff3eb}.unsure-option{grid-column:1/-1;min-height:44px;padding:10px 16px;font-size:14px;gap:10px;justify-content:center}.unsure-option small{font-size:11px;color:var(--muted)}.lesson-answer .feedback{min-height:24px;margin-top:12px;font-size:13px;padding:0}.lesson-answer .feedback:empty{margin-top:12px;min-height:24px}.numeric-form{display:flex;align-items:flex-end;gap:10px;max-width:560px}.numeric-field{display:grid;gap:9px;flex:1;min-width:0;font-size:12px;color:var(--muted)}.numeric-field .numeric{width:100%;min-width:0;max-width:none;margin:0;height:46px}.numeric-confirm,.numeric-unsure{border:1px solid var(--line);background:white;border-radius:9px;min-height:46px;padding:11px 14px;font-size:14px}.numeric-confirm{background:var(--accent);border-color:var(--accent);color:#fff;min-width:46px;display:grid;place-items:center}.numeric-unsure[aria-pressed=true]{background:var(--tint);border-color:var(--accent)}.numeric-hint{color:var(--muted);font-size:12px;line-height:1.7;margin-top:9px}.lesson-page .explanation-pane{margin-top:28px}.teaching-guide{position:sticky;top:0;z-index:5;background:var(--paper);padding-top:8px}.step-nav{display:grid;grid-template-columns:repeat(3,minmax(0,1fr));gap:8px}.step-tab{position:relative;display:flex;flex-direction:column;justify-content:center;gap:5px;border:1px solid var(--line);border-radius:10px 10px 0 0;background:var(--surface);padding:17px 38px 17px 20px;min-width:0;min-height:88px;text-align:left}.step-name{display:flex;align-items:baseline;gap:10px;line-height:1.5;font-size:16px}.step-number{font:17px Georgia,serif;color:var(--muted)}.step-en{font-size:12px;color:var(--muted);padding-left:20px;line-height:1.4;overflow-wrap:anywhere}.step-arrow{position:absolute;right:16px;top:22px;font-size:22px;font-weight:400;color:var(--accent)}.step-tab[aria-current=step]{color:var(--accent);border-color:var(--accent);background:var(--tint)}.step-tab[aria-current=step] .step-number{color:var(--accent)}.step-tab[aria-current=step]:after{content:"";position:absolute;left:24px;bottom:-9px;height:8px;border-left:1px solid var(--accent)}.step-tab:disabled{opacity:.55}.step-tab:not(:disabled):hover{border-color:var(--accent)}.explanation-row{display:flex;gap:18px;align-items:flex-start;background:var(--tint);margin-top:8px;padding:20px 24px;border-radius:0 0 10px 10px;min-height:114px}.explanation-row .explainer-text{padding:0;margin:0;flex:1;min-width:0}.explanation-row .zh{font-size:16px;line-height:1.8;margin:0}.explanation-row .en{font-size:14px;line-height:1.6;margin:6px 0 0;color:var(--muted)}.read-step{width:44px;height:44px;flex-shrink:0;display:grid;place-items:center;color:var(--accent);border:1px solid color-mix(in srgb,var(--accent) 25%,transparent);border-radius:50%;background:var(--surface)}.read-step.active{background:var(--accent);color:white}.lesson-page .teaching-stage{margin-top:14px;background:linear-gradient(145deg,var(--tint),var(--surface));min-height:420px}.lesson-page .visual{max-width:760px;margin:auto;padding-top:16px}.lesson-page .visual svg{max-height:310px}.lesson-page .formula-stage{padding:25px 20px}.lesson-page .formula-history:empty{display:none}.lesson-completion{color:var(--muted);font-size:12px;line-height:1.7;min-height:21px;margin-top:13px}.lesson-page .lesson-bottom{margin-top:28px}.original-option:focus-visible,.step-tab:focus-visible{outline-offset:2px}
@media(max-width:760px){.lesson-page .lesson-top{align-items:flex-start;gap:12px}.lesson-page .question-pane{padding:20px}.lesson-page .question-body{font-size:17px;line-height:1.85}.lesson-page .explanation-pane{margin-top:22px}.lesson-options{grid-template-columns:1fr;gap:8px}.original-option{padding:12px 14px;min-height:48px}.unsure-option{min-height:44px}.choice-content{font-size:16px}.step-nav{gap:5px}.step-tab{padding:13px 9px;min-height:105px;justify-content:flex-start;gap:7px}.step-name{font-size:13px;gap:6px;align-items:baseline}.step-number{font-size:14px}.step-name>span:last-child{flex:1;min-width:0}.step-en{padding-left:0;font-size:10px;line-height:1.5;padding-bottom:16px}.step-arrow{top:auto;bottom:4px;right:9px;font-size:18px}.step-tab[aria-current=step]:after{left:16px}.explanation-row{padding:15px 16px;gap:10px;min-height:130px}.explanation-row .zh{font-size:15px}.explanation-row .en{font-size:13px}.read-step{width:44px;height:44px}.lesson-page .teaching-stage{min-height:330px}.lesson-page .visual{padding:12px 12px 0}.lesson-page .visual svg{max-height:260px}.numeric-form{gap:7px}.numeric-unsure{padding-left:10px;padding-right:10px}.numeric-hint{font-size:11px}.lesson-page .source-image button{padding:8px}}
@media(max-width:370px){.lesson-page .question-pane{padding:16px}.lesson-page .question-body{font-size:16px}.step-tab{padding-left:8px;padding-right:8px}.step-name{font-size:12px;gap:5px}.explanation-row{padding:14px 12px}.explanation-row .zh{font-size:14px}.explanation-row .en{font-size:12px}.lesson-page .formula-stage{padding-left:10px;padding-right:10px}}