.sidebar{
  background-color: #015293;
}
.app{
     background: url('http://ydyl.acfic.org.cn/ydyl/2020/03/2020031915123725720.png') top center no-repeat;
     background-size:100% ;
}
@media (max-width: 991px){
  .sidebar {
      position: fixed;
      width: 302px;
      height: 100%;
      margin-left: -320px;
  }
}