判断样式文件是否下载

      var _themeLink = 'http://xxxxxx.com/xxx/xx.css';
      if(!_themeLink) return;
      var ajax = new XMLHttpRequest();
      ajax.open('get',_themeLink.href);
      ajax.send();
      ajax.onreadystatechange = ()=>{
        if(ajax.readyState==4&&ajax.status==200){
          this.loaded = 1;
          if(this.cssIsReady){
            this.cssIsReady();
          }
        }
      }