body{
    background-image: url(
        https://images.cnblogs.com/cnblogs_com/timespry/2214801/o_220908070803_%E5%BE%AE%E4%BF%A1%E5%9B%BE%E7%89%87_20220908150709.jpg);
    background-attachment: fixed;
    background-repeat: no-repeat;
    background-size: cover;
    
    opacity:0.8; //0.0到1随便设置，越低越透明
}