引用：https://blog.csdn.net/Sabrina_cc/article/details/125871591

子菜单选中后，文字和背景改变：

 .el-submenu__title i{
   color: #e8e8e8 !important;
 }
#app .sidebar-container .theme-dark .nest-menu .el-submenu .is-active > .el-submenu__title, #app .sidebar-container .theme-dark .el-submenu .el-menu-item.is-active{
  background-color: #035D87 !important;
  color: #fff !important;
}