* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    font-family: Arial, sans-serif;
    min-height: 100vh;
    margin: 0;
    padding: 0;
    background:
        radial-gradient(circle at 20% 20%, #7c3aed 0%, transparent 30%),
        radial-gradient(circle at 80% 80%, #c026d3 0%, transparent 30%),
        radial-gradient(circle at 70% 10%, #4c1d95 0%, transparent 25%),
        linear-gradient(135deg, #09001a, #1e0738, #12002b);
        align-items:center;
}
nav{
    width:100%;
    margin:0;
    padding:0;
    background:#000000;
}

nav ul{
    margin:0;
    padding:0;
    list-style:none;
    display:flex;
    width:100%;
}

nav li{
    flex:1;
    text-align:center;
}
/* Tiêu đề */
h1 {
    text-align: center;
    color: #f5eaff;
    font-size: 45px;
    margin-bottom: 20px;
    letter-spacing: 3px;

    text-shadow:
        0 0 10px #a855f7,
        0 0 25px #9333ea,
        0 0 40px #c026d3;
}

/* Nút về trang chủ */
body > a {
    display: block;
    width: 170px;
    margin: 0 auto 35px;
    padding: 12px;

    text-align: center;
    text-decoration: none;

    color: #ffffff;
    background: linear-gradient(135deg, #7c3aed, #c026d3);

    border-radius: 12px;

    font-weight: bold;

    box-shadow: 0 0 15px rgba(168, 85, 247, 0.5);

    transition: 0.3s;
}

body > a:hover {
    transform: translateY(-4px);

    box-shadow:
        0 0 20px #a855f7,
        0 0 35px #c026d3;
}

/* Khung danh sách */
ol {
    width: 430px;
    margin: auto;
    padding: 25px 40px;

    background: #111827;

    border: 1px solid #374151;
    border-radius: 20px;

    box-shadow:
        0 15px 40px rgba(0, 0, 0, 0.5),
        0 0 25px rgba(168, 85, 247, 0.2);
}

/* Từng bài tập */
li {
    margin: 12px 0;
    padding: 13px 16px;

    background: #1f2937;

    border-left: 4px solid #a855f7;
    border-radius: 9px;

    transition: 0.3s;
}

/* Hiệu ứng khi rê chuột */
li:hover {
    background: #374151;

    transform: translateX(8px);

    box-shadow:
        0 0 15px rgba(168, 85, 247, 0.4);
}

/* Link bài tập */
li a {
    color: #e5e7eb;

    text-decoration: none;

    font-size: 18px;
    font-weight: bold;

    transition: 0.3s;
}

/* Hover link */
li a:hover {
    color: #d8b4fe;

    text-shadow:
        0 0 8px #a855f7;
}