body { 
     background-color: #efefef;
     background-image: url(https://cdn.luogu.com.cn/upload/image_hosting/yjcoadsr.png);
     background-repeat: no-repeat; 
     background-attachment: fixed; 
     background-position: center 0; 
     background-size: cover; 
     opacity: 0.8;
  }