教程：

放到后台自定义CSS

/*手机侧边栏背景图片*/
@media (max-width: 767px){
.mobile-navbar.show,.mobile-navbar.left{
    background-size: cover;
    background-repeat: no-repeat;
    background-position: center center;
    cursor: pointer;
  background-image:linear-gradient(rgba(255, 255,255,0),rgba(255,255,255, 0.3)),url(图片链接);}
  .mobile-nav-widget .box-body {
    background: var(--muted-border-color) !important;}
}