:root {
            --sidebar-bg: #1e1e28;
            --sidebar-text: #e0e0e0;
            --sidebar-text-hover: #ffffff;
            --sidebar-item-hover-bg: rgba(255, 255, 255, 0.1);
            --sidebar-item-active-bg: #4285f4; 
            --sidebar-item-active-text: #ffffff;
            --main-bg: #f5f5f7;
            --main-text: #333333;
            --player-shadow: rgba(0, 0, 0, 0.2);
            --transition-speed: 0.3s;
        }

        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
            font-family: 'Segoe UI', 'PingFang SC', 'Microsoft YaHei', sans-serif;
        }

        body {
            background-color: var(--main-bg);
            color: var(--main-text);
            min-height: 100vh;
            display: flex; /* 使用 Flexbox 布局 */
            overflow-x: hidden;
        }

        /* --- 左侧导航栏 --- */
        .sidebar {
            width: 260px;
            background-color: var(--sidebar-bg);
            color: var(--sidebar-text);
            height: 100vh;
            position: fixed;
            top: 0;
            left: 0;
            overflow-y: auto;
            padding: 20px 0;
            box-shadow: 2px 0 10px var(--player-shadow);
            z-index: 1000;
        }

        .sidebar-header {
            padding: 0 20px 20px 20px;
            border-bottom: 1px solid rgba(255, 255, 255, 0.05);
            margin-bottom: 15px;
        }

        .sidebar-header h2 {
            font-size: 1.4rem;
            font-weight: 600;
            color: #ffffff;
        }

        .sidebar-menu {
            list-style: none;
        }

        /* 一级菜单项 */
        .sidebar-item-level1 {
            padding: 0;
        }

        .sidebar-item-level1 > a {
            display: block;
            padding: 12px 20px;
            color: var(--sidebar-text);
            text-decoration: none;
            font-weight: 500;
            font-size: 1rem;
            transition: background-color var(--transition-speed), color var(--transition-speed);
            position: relative;
            cursor: pointer;
        }

        .sidebar-item-level1 > a:hover {
            background-color: var(--sidebar-item-hover-bg);
            color: var(--sidebar-text-hover);
        }

       
        .sidebar-item-level1 > a::after {
            content: '▼';
            font-size: 0.7rem;
            position: absolute;
            right: 20px;
            transition: transform var(--transition-speed);
        }
        .sidebar-item-level1.active > a::after {
            transform: rotate(180deg);
        }
       

        /* 二级菜单容器 */
        .sidebar-submenu {
            list-style: none;
            padding-left: 0;
            max-height: 0;
            overflow: hidden;
            transition: max-height 0.3s ease-out;
        }

        .sidebar-item-level1.active > .sidebar-submenu {
            max-height: 1000px; /* 足够大以展开所有内容 */
        }

        /* 二级菜单项 */
        .sidebar-item-level2 {
            padding: 0;
        }

        .sidebar-item-level2 a {
            display: block;
            padding: 10px 20px 10px 35px; /* 左侧缩进 */
            color: var(--sidebar-text);
            text-decoration: none;
            font-weight: 400;
            font-size: 0.95rem;
            transition: all var(--transition-speed);
            position: relative;
            overflow: hidden;
        }

        .sidebar-item-level2 a:hover {
            background-color: var(--sidebar-item-hover-bg);
            color: var(--sidebar-text-hover);
        }

        /* 激活状态的二级菜单项 */
        .sidebar-item-level2 a.active {
            background-color: var(--sidebar-item-active-bg);
            color: var(--sidebar-item-active-text);
        }

        /* 激活项左侧的小竖条 */
        .sidebar-item-level2 a.active::before {
            content: '';
            position: absolute;
            left: 0;
            top: 0;
            bottom: 0;
            width: 4px;
            background-color: var(--sidebar-item-active-text);
        }


        /* --- 主内容区域 --- */
        .main-content {
            flex-grow: 1; /* 占据剩余空间 */
            margin-left: 260px; /* 与侧边栏同宽 */
            padding: 30px;
        }

        .page-title {
            margin-bottom: 25px;
            font-size: 1.8rem;
            font-weight: 700;
            color: #212121;
        }

        /* 视频标题 */
        #current-video-title {
            font-size: 1.3rem;
            font-weight: 600;
            margin-bottom: 15px;
            color: #424242;
        }

        /* 播放器容器 */
        .player-container {
            background: #000;
            border-radius: 10px;
            overflow: hidden;
            box-shadow: 0 5px 15px var(--player-shadow);
            max-width: 1000px; /* 限制最大宽度 */
            margin: 0 auto;
            position: relative; /* 用于定位加载动画 */
        }

        .video-js {
            width: 100%;
            height: 0;
            padding-top: 56.25%; /* 16:9 宽高比 */
        }

        /* 加载动画 */
        .loader {
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
            width: 50px;
            height: 50px;
            z-index: 5;
            opacity: 0;
            transition: opacity var(--transition-speed);
        }

        .loader.active {
            opacity: 1;
        }

        .loader-circle {
            position: absolute;
            width: 100%;
            height: 100%;
            border: 3px solid transparent;
            border-top: 3px solid var(--sidebar-item-active-bg);
            border-radius: 50%;
            animation: spin 1s linear infinite;
        }

        /* 响应式设计 */
        @media (max-width: 768px) {
            body {
                flex-direction: column; /* 小屏幕上下排列 */
            }
            .sidebar {
                width: 100%;
                height: auto;
                position: relative;
                box-shadow: 0 2px 5px var(--player-shadow);
            }
            .sidebar-header {
                 text-align: center;
                 padding: 15px 0;
                 margin-bottom: 10px;
            }
            .main-content {
                margin-left: 0;
                padding: 20px 15px;
            }
            .page-title {
                font-size: 1.5rem;
                text-align: center;
            }
            #current-video-title {
                font-size: 1.1rem;
                text-align: center;
            }
            .player-container {
                border-radius: 8px;
            }
        }

        @keyframes spin {
            0% { transform: translate(-50%, -50%) rotate(0deg); }
            100% { transform: translate(-50%, -50%) rotate(360deg); }
        }