@import "https://fonts.googleapis.com/css2?family=Roboto:wght@300;400;500;700&display=swap";:root{--bg-color:#f9f9f9;--bg-secondary:#fff;--text-main:#333;--text-dim:#666;--border-color:#e0e0e0;--accent-color:#0056b3;--accent-hover:#004494;--accent-green:#28a745;--accent-red:#dc3545}*{box-sizing:border-box;margin:0;padding:0;font-family:Roboto,sans-serif}body{background-color:var(--bg-color);color:var(--text-main);min-height:100vh;line-height:1.6}.container{max-width:1000px;margin:0 auto;padding:2rem 1rem}header{text-align:center;border-bottom:2px solid var(--border-color);margin-bottom:2.5rem;padding-bottom:1rem}header h1{color:var(--accent-color);margin-bottom:.5rem;font-size:2.5rem}header p{color:var(--text-dim);font-size:1.1rem}.card{background:var(--bg-secondary);border:1px solid var(--border-color);color:var(--text-main);border-radius:8px;padding:24px;text-decoration:none;transition:box-shadow .2s,transform .2s;box-shadow:0 2px 4px #0000000d}.card:hover{transform:translateY(-2px);box-shadow:0 4px 12px #0000001a}.subject-grid,.chapter-grid{grid-template-columns:repeat(auto-fit,minmax(280px,1fr));gap:1.5rem;display:grid}.card-title{color:var(--accent-color);align-items:center;gap:10px;margin-bottom:.5rem;font-size:1.4rem;display:flex}.btn-back{color:var(--accent-color);cursor:pointer;background:0 0;border:none;align-items:center;gap:8px;margin-bottom:1.5rem;font-size:1rem;font-weight:500;display:inline-flex}.btn-back:hover{text-decoration:underline}.btn{border:1px solid var(--accent-color);cursor:pointer;color:var(--accent-color);background:0 0;border-radius:4px;justify-content:center;align-items:center;gap:8px;padding:10px 20px;font-size:1rem;font-weight:500;transition:all .2s;display:inline-flex}.btn:hover:not(:disabled){background:var(--accent-color);color:#fff}.btn:disabled{opacity:.5;cursor:not-allowed;border-color:var(--border-color);color:var(--text-dim)}.btn-primary{background:var(--accent-color);color:#fff}.btn-primary:hover:not(:disabled){background:var(--accent-hover)}.flashcard-wrapper{perspective:1000px;width:100%;max-width:600px;height:350px;margin:0 auto 2rem}.flashcard-inner{text-align:center;width:100%;height:100%;transform-style:preserve-3d;cursor:pointer;transition:transform .6s cubic-bezier(.4,0,.2,1);position:relative}.flashcard-wrapper.flipped .flashcard-inner{transform:rotateY(180deg)}.flashcard-front,.flashcard-back{backface-visibility:hidden;background:var(--bg-secondary);border:1px solid var(--border-color);border-radius:8px;flex-direction:column;justify-content:center;align-items:center;width:100%;height:100%;padding:2rem;font-size:1.2rem;display:flex;position:absolute;box-shadow:0 4px 10px #0000000d}.flashcard-back{border:2px solid var(--accent-color);background:#fdfdfd;transform:rotateY(180deg)}.flashcard-controls{justify-content:center;gap:1rem;display:flex}.quiz-container{max-width:700px;margin:0 auto}.quiz-header{color:var(--text-dim);justify-content:space-between;margin-bottom:1.5rem;font-size:1.1rem;font-weight:500;display:flex}.quiz-question h3{color:var(--text-main);margin-bottom:1.5rem;font-size:1.4rem}.quiz-options{flex-direction:column;gap:1rem;display:flex}.quiz-option{background:var(--bg-secondary);border:1px solid var(--border-color);text-align:left;color:var(--text-main);cursor:pointer;border-radius:4px;justify-content:space-between;align-items:center;gap:12px;padding:1rem 1.5rem;font-size:1rem;transition:all .2s;display:flex}.quiz-option:hover:not(:disabled){border-color:var(--accent-color);background:#f0f8ff}.quiz-option.correct{border-color:var(--accent-green);color:var(--accent-green);background:#e8f5e9;font-weight:500}.quiz-option.wrong{border-color:var(--accent-red);color:var(--accent-red);background:#ffebee}.quiz-option:disabled{cursor:default}.quiz-feedback{border:1px solid var(--border-color);background:#f9f9f9;border-radius:4px;margin-top:1.5rem;padding:1rem}.quiz-feedback .explanation{color:var(--text-main);margin-top:10px;font-size:.95rem;line-height:1.5}.feedback-text{font-size:1.1rem;font-weight:700}.feedback-text.correct{color:var(--accent-green)}.feedback-text.wrong{color:var(--accent-red)}.progress-bar-container{background:#e0e0e0;border-radius:4px;width:100%;height:8px;margin-bottom:2rem;overflow:hidden}.progress-bar{background:var(--accent-color);height:100%;transition:width .3s}@media (width<=768px){.subject-grid,.chapter-grid{grid-template-columns:1fr}}