添加渐变背景

body::before {
    content: '';
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index: -520;
    pointer-events: none;
}
body::before {
    background: linear-gradient( 90deg, rgba(247, 149, 51, .1), rgba(243, 112, 85, .1) 15%, rgba(239, 78, 123, .1) 30%, rgba(161, 102, 171, .1) 44%, rgba(80, 115, 184, .1) 58%, rgba(16, 152, 173, .1) 72%, rgba(7, 179, 155, .1) 86%, rgba(109, 186, 130, .1)); /*背景颜色*/
}

添加背景图片

body::before {
    content: '';
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index: -520;
    pointer-events: none;
}
body::before {
    background-image: url(/img/1.avif);/* 这里设置图片 */
    background-repeat: no-repeat;
    background-size: cover;
}

添加动态渐变背景

Demo站：https://cnmp.net

body::before {
    content: '';
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    z-index: -520;
    pointer-events: none;
}
body::before {
    background: linear-gradient( 90deg, rgba(247, 149, 51, .1), rgba(243, 112, 85, .1) 15%, rgba(239, 78, 123, .1) 30%, rgba(161, 102, 171, .1) 44%, rgba(80, 115, 184, .1) 58%, rgba(16, 152, 173, .1) 72%, rgba(7, 179, 155, .1) 86%, rgba(109, 186, 130, .1)); /*背景颜色*/
    background-size: 500%;
    animation: bgAnimation 15s linear infinite; /*执行动画*/
}
@keyframes bgAnimation{
    0%{
        background-position: 0% 50%;
    }
    50%{
        background-position: 100% 50%;
    }
    100%{
        background-position: 0% 50%;
    }
}