:root{--primary:#5b6af0;--primary-dark:#4451d6;--success:#34c77b;--error:#f05b5b;--bg:#f0f4ff;--card-bg:#fff;--text:#2c2c4a;--muted:#7a7a9d;--radius:16px;--shadow:0 4px 20px #5b6af01a}*{box-sizing:border-box;margin:0;padding:0}body{background:var(--bg);color:var(--text);flex-direction:column;min-height:100vh;padding:24px 16px 48px;font-family:Segoe UI,system-ui,sans-serif;display:flex}#app{flex-direction:column;flex:1;gap:20px;width:100%;max-width:680px;margin:0 auto;display:flex}.app-header{text-align:center;padding:12px 0}.app-header h1{color:var(--primary);font-size:2rem}.subtitle{color:var(--muted);margin-top:4px}.card{background:var(--card-bg);border-radius:var(--radius);box-shadow:var(--shadow);padding:24px}.problem-input label,.method-selector label{margin-bottom:12px;font-size:1.05rem;font-weight:600;display:block}.problem-row{flex-wrap:wrap;align-items:center;gap:12px;display:flex}input[type=number]{text-align:center;width:80px;color:var(--text);border:2px solid #d4d8f7;border-radius:12px;outline:none;padding:12px;font-size:1.5rem;font-weight:700;transition:border-color .2s}input[type=number]:focus{border-color:var(--primary)}.operator{color:var(--primary);font-size:1.8rem;font-weight:700}select{color:var(--text);cursor:pointer;background:#fff;border:2px solid #d4d8f7;border-radius:12px;outline:none;padding:10px 16px;font-size:1rem;transition:border-color .2s}select:focus{border-color:var(--primary)}button{cursor:pointer;border:none;border-radius:12px;padding:12px 24px;font-size:1rem;font-weight:600;transition:background .2s,transform .1s}button:active{transform:scale(.97)}button:disabled{opacity:.4;cursor:not-allowed}#btnStart{background:var(--primary);color:#fff;padding:12px 28px;font-size:1.1rem}#btnStart:hover:not(:disabled){background:var(--primary-dark)}.method-buttons{flex-wrap:nowrap;gap:10px;display:flex}.method-btn{aspect-ratio:1;min-width:0;color:var(--text);text-align:center;cursor:pointer;background:#eef0fc;border:2px solid #0000;border-radius:14px;flex-direction:column;flex:1 1 0;justify-content:center;align-items:center;gap:6px;padding:8px 4px;font-size:.8rem;font-weight:600;line-height:1.15;transition:background .2s,border-color .2s,color .2s,transform .1s;display:flex}.method-btn:hover{background:#d9dcf9}.method-btn.is-active{border-color:var(--primary);color:var(--primary);background:#fff;box-shadow:0 4px 14px #5b6af033}.method-btn .method-icon{font-size:1.7rem;line-height:1}.method-btn .method-name{font-size:.78rem}@media (width<=480px){.method-btn{padding:6px 2px;font-size:.7rem}.method-btn .method-icon{font-size:1.4rem}.method-btn .method-name{font-size:.68rem}}.setup-cards{flex-direction:column;gap:20px;display:flex}.setup-summary{background:var(--card-bg);border-radius:var(--radius);width:100%;box-shadow:var(--shadow);color:var(--primary);cursor:pointer;border:none;justify-content:space-between;align-items:center;padding:12px 20px;font-size:1.1rem;font-weight:700;display:flex}.setup-summary:hover{background:#f6f8ff}.setup-summary-problem{letter-spacing:.5px}.setup-summary-toggle{width:28px;height:28px;color:var(--primary);background:#eef0fc;border-radius:50%;justify-content:center;align-items:center;transition:transform .2s,background .2s;display:inline-flex}.setup-summary:hover .setup-summary-toggle{background:#d9dcf9}.setup-summary[aria-expanded=true] .setup-summary-toggle{transform:rotate(180deg)}.hidden{display:none!important}.lesson-header{justify-content:space-between;align-items:center;margin-bottom:16px;display:flex}.problem-display{color:var(--primary);font-size:1.4rem;font-weight:700}.progress-display{color:var(--muted);font-size:.9rem}.step-title{color:var(--text);margin-bottom:20px;font-size:1.25rem}.step-container{border:2px dashed #d4d8f7;border-radius:12px;min-height:200px;margin-bottom:20px;padding:24px;transition:background .3s}.step-container.feedback-correct{border-color:var(--success);background:#e8fff4}.step-container.feedback-wrong{border-color:var(--error);background:#fff0f0}.step-placeholder{text-align:center;color:var(--muted);padding:32px 0;font-size:1.1rem}.step-placeholder .step-url{opacity:.6;margin-top:8px;font-size:.8rem}.controls{flex-wrap:wrap;gap:10px;display:flex}#btnPrev,#btnNext{color:var(--primary);background:#eef0fc}#btnPrev:hover:not(:disabled),#btnNext:hover:not(:disabled){background:#d9dcf9}#btnCheck{background:var(--success);color:#fff}#btnCheck:hover{background:#29b36c}#btnShowAnswer{color:#e07a00;background:#fff3e0}#btnShowAnswer:hover{background:#ffe0b2}.app-footer{text-align:center;margin-top:auto;padding-top:16px}.footer-text{color:var(--muted);font-size:.85rem}