下方代码放后台-自定义CSS

/* 背景图片_[猫之书_mzhis.com] */
    body::before {
        content: '';
position: fixed;
        top: 0;
        left: 0;
        right: 0;
        bottom: 0;
      background: url(图片地址);
        background-position: center 0;
        background-size: cover;
        z-index: -1;
        opacity: 0.75;
    }