:root{--bg: #f8fafc;--sidebar-bg: #ffffff;--primary: #2563eb;--text: #0f172a;--muted: #64748b;--border: #e5e7eb}*{box-sizing:border-box}body{margin:0;font-family:Inter,system-ui,-apple-system,BlinkMacSystemFont,Segoe UI,sans-serif;background:var(--bg);color:var(--text)}.header{height:60px;background:var(--sidebar-bg);border-bottom:1px solid var(--border);display:flex;align-items:center;gap:14px;padding:0 16px;position:sticky;top:0;z-index:50}.menu-btn{font-size:22px;background:none;border:none;cursor:pointer}.header-title{font-size:18px;font-weight:600}.layout{height:100vh;display:flex;flex-direction:column}.content{flex:1;display:flex;overflow:hidden}.sidebar{width:300px;background:var(--sidebar-bg);border-right:1px solid var(--border);overflow-y:auto;padding:12px;transition:transform .3s ease}.sidebar-section{margin-bottom:10px}.sidebar-section-title{width:100%;background:none;border:none;padding:10px;font-size:14px;font-weight:600;color:var(--text);cursor:pointer;display:flex;justify-content:space-between;border-radius:6px}.sidebar-section-title:hover{background:#f1f5f9}.sidebar-items{list-style:none;margin:6px 0 0;padding-left:8px}.sidebar-item{width:100%;background:none;border:none;padding:8px 10px;font-size:14px;text-align:left;cursor:pointer;border-radius:6px;color:var(--muted)}.sidebar-item:hover{background:#f1f5f9;color:var(--text)}.sidebar-item.active{background:#e0e7ff;color:var(--primary);font-weight:600}.main-content{flex:1;padding:20px;overflow-y:auto}.video-section{max-width:960px;margin:0 auto}.video-title{font-size:24px;font-weight:700;margin-bottom:16px}.video-wrapper{position:relative;width:100%;padding-top:56.25%;background:#000;border-radius:12px;overflow:hidden;box-shadow:0 10px 30px #00000026}.video-wrapper video{position:absolute;inset:0;width:100%;height:100%}.video-placeholder{max-width:600px;margin:80px auto;text-align:center}.video-placeholder h2{font-size:28px;margin-bottom:10px}.video-placeholder p{color:var(--muted)}.progress-bar{height:28px;background:#e5e7eb;position:relative}.progress-fill{height:100%;background:#2563eb;transition:width .3s ease}.progress-bar span{position:absolute;inset:0;display:flex;align-items:center;justify-content:center;font-size:13px;font-weight:600;color:#0f172a}.lesson-header{position:sticky;top:-60;background:#f8fafc;padding:12px 0;z-index:10;display:flex;justify-content:space-between;align-items:center}.complete-btn{border:1px solid #2563eb;background:#fff;color:#2563eb;padding:6px 12px;border-radius:6px;cursor:pointer;font-weight:600}.complete-btn.done{background:#2563eb;color:#fff}.lesson-nav{display:flex;justify-content:space-between;margin-top:24px}.lesson-nav button{padding:10px 16px;border-radius:8px;border:1px solid #e5e7eb;background:#fff;cursor:pointer;font-weight:600}.lesson-nav button:disabled{opacity:.5;cursor:not-allowed}.sidebar-item.completed{color:#16a34a}@media(max-width:768px){.sidebar{position:fixed;top:60px;bottom:0;transform:translate(-100%);z-index:100;box-shadow:4px 0 20px #0000001a}.sidebar.open{transform:translate(0)}.main-content{padding:14px}.video-title{font-size:20px}}