{技术操作}通过类名切换背景图

原本：<div id="app" :class="{'active_bg':!isActive}" >
现在：<div id="app" :class="{'active_bg':!isActive,'active_bg_new':isActive}" >
通过类名切换不同标签下的背景图
css:
.active_bg {
  z-index:100;
  background: url("./assets/bg3.png") ;
  background-size: 100% 100% !important;
  background-repeat: no-repeat !important;
}
.active_bg_new {
  z-index:100;
  background: url("./assets/feiyueyuanqu/bg_all.png") !important;
  background-size: 100% 100% !important;
  background-repeat: no-repeat !important;
}