仅作记录，我也忘了从哪里抄过来的了

    <style>
      body {
        /* 背景图片地址,替换成你喜欢的 */
        background-image: url("https://666.16283684.xyz/file/AgACAgQAAyEGAASJsrt2AAM8aM1-sChyjZwJsMGQYWH6ahmvv4QAAo_QMRtmRmlSnfuSnlk-WP0BAAMCAAN3AAM2BA.jpg");
        background-size: cover;
        background-position: center center;
        background-repeat: no-repeat;
        background-attachment: fixed;
      }
      .layout {
        background-color: transparent;
      }
    </style>