body {
    position: relative;
}

body::before {
    content: "";
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-image: url("/res/bg.png");
    background-size: cover;
    background-position: center;
    z-index: -1; /* 放在内容后面 */
    opacity: 1; 
}