/* Global Styles */
        html {
            scroll-behavior: smooth; /* Tạo hiệu ứng cuộn mượt khi click menu */
        }
        
        body {
            font-family: Arial, sans-serif;
            line-height: 1.6;
            margin: 0;
            padding: 20px;
            background-color: #f0f0f0;
            padding-top: 80px; /* Tránh menu cố định đè lên nội dung */
        }

        /* --- Menu điều hướng nhanh --- */
        .quick-nav {
            position: fixed;
            top: 0;
            left: 0;
            width: 100%;
            background: #333;
            color: white;
            padding: 10px 0;
            z-index: 1000;
            box-shadow: 0 2px 5px rgba(0,0,0,0.2);
            text-align: center;
        }

        .quick-nav a {
            color: white;
            text-decoration: none;
            margin: 0 15px;
            font-weight: bold;
            font-size: 14px;
            transition: color 0.3s;
        }

        .quick-nav a:hover {
            color: #ffc;
        }

        .exercise-container {
            background: white;
            padding: 20px;
            margin-bottom: 50px;
            border: 1px solid #ccc;
            border-radius: 8px;
            position: relative;
        }

        h1.main-title {
            text-align: center;
            color: #333;
            border-bottom: 2px solid #333;
            padding-bottom: 10px;
        }

        /* --- CSS cho Bài 1, 2, 3 --- */
        .box-message {
            border: #00F solid 2px;
            width: 90%;
            max-width: 700px;
            margin: 20px auto;
            background-color: #FFC;
            border-radius: 10px;
            padding: 10px;
        }

        .box-lesson {
            border: #00F solid 2px;
            width: 90%;
            max-width: 450px;
            min-height: 330px;
            margin: 20px auto;
            background: #444 linear-gradient(rgba(0,0,0,0.5), rgba(0,0,0,0.5));
            background-size: cover;
            border-radius: 10px;
            padding: 10px;
            color: #FFC;
        }

        .title-red { color: #900; text-align: center; }
        .title-yellow { color: #CF0; text-align: center; }
        
        .content-p {
            font-size: 18px;
            padding: 0 20px;
            text-align: justify;
            text-indent: 50px;
        }

        /* --- CSS cho Bài 4 --- */
        #Wrapper {
            width: 100%;
            max-width: 960px;
            margin: 20px auto;
            border: 1px solid black;
            height: 600px;
        }
        #Tren {
            height: 30%;
            background: green;
            display: flex;
            align-items: center;
            justify-content: center;
            color: white;
            font-weight: bold;
        }
        #Duoi {
            height: 70%;
            background: blue;
            display: flex;
            align-items: center;
            justify-content: center;
            color: white;
            font-weight: bold;
        }

        /* --- CSS cho Bài 5 --- */
        .container-float {
            width: 900px;
            border: 3px solid blue;
            margin: 20px auto;
            padding: 10px;
            overflow: hidden;
        }
        .item {
            border: 3px solid #666;
            margin: 12px 6px;
            padding: 12px 6px;
            background-color: #EFC0DE;
        }
        #fruit { width: 45%; float: left; }
        #computer { width: 45%; float: right; }
        #stationary { width: 96%; clear: both; margin-top: 20px; }
        .justify-text { text-align: justify; }

        /* --- CSS cho Bài tập tại lớp 1 --- */
        .layout-1-header {
            background-color: #666;
            color: white;
            padding: 30px;
            text-align: center;
            font-size: 35px;
        }
        .layout-1-wrapper { display: flex; width: 100%; height: 400px; }
        .layout-1-nav {
            width: 30%;
            background: #ccc;
            padding: 20px;
        }
        .layout-1-article {
            width: 70%;
            background: #eee;
            padding: 20px;
        }
        .layout-1-footer {
            background: #777;
            color: white;
            text-align: center;
            padding: 10px;
            clear: both;
        }

        /* --- CSS cho Bài tập tại lớp 2 --- */
        .qh-header { background-color: #FCC; padding: 20px; text-align: center; }
        .qh-container {
            display: flex;
            background-color: #bbb;
            padding: 10px;
            justify-content: space-around;
        }
        .qh-column {
            width: 30%;
            background: #ddd;
            padding: 10px;
        }
        .qh-column img {
            width: 100%;
            height: 150px;
            object-fit: cover;
            margin-top: 10px;
        }
        .qh-footer { background-color: #FCC; padding: 15px; text-align: center; font-size: 12px; }

        /* --- CSS cho Bài tập tại lớp 3 --- */
        .complex-wrapper { width: 1000px; margin: auto; border: 1px solid #ccc; }
        .c-header { background: #eee; padding: 40px; text-align: center; font-size: 30px; font-weight: bold; }
        .c-banner { background: #cff; padding: 20px; text-align: center; font-weight: bold; }
        .c-nav { background: #9bb; padding: 15px; font-weight: bold; }
        .c-main { display: flex; height: 300px; }
        .c-sidebar-l { width: 25%; background: #aaa; padding: 10px; }
        .c-content { width: 50%; background: #ccc; padding: 10px; }
        .c-sidebar-r { width: 25%; background: #bbb; padding: 10px; }
        .c-footer { background: #eee; text-align: center; padding: 10px; border-top: 1px solid #999; }