我从bootswatch下载了主题，并试图允许用户切换主题。切换主题后，所有引导CSS都会暂时消失，直到加载新主题为止。在加载CSS之前如何防止更改，或者在加载新主题之前切换回默认主题？

index.ejs（在头）

    <link rel="stylesheet" href="/external/bootstrap/css/bootstrap_yeti.min.css"
          ng-if="myTheme == ''">
    <link rel="stylesheet" ng-href="/external/bootstrap/css/bootstrap_{{myTheme}}.min.css"
          ng-if="myTheme != ''">

选拔

    <select class="form-control"
            id="theme"
            name="theme"
            ng-model="theme"
            ng-change="newTheme()">
      <option value="" disabled>Select Theme</option>
      <option ng-repeat="(key, val) in availableThemes" value="{{val}}">{{key}}</option>
    </select>

索引控制器

    $scope.myTheme = '';
    $rootScope.$watch('myTheme', function(value) {
      if (value != undefined) {
        $scope.myTheme = value;
      }
    });

选择控制器

$scope.availableThemes = {
  Cerulean: 'cerulean',
  Cosmo:    'cosmo',
  Yeti:     'yeti'
};
$scope.newTheme = function() {
  console.log($scope.theme);
  if ($scope.theme != undefined) {
    $rootScope.myTheme = $scope.theme;
  }
}

任何帮助表示赞赏！谢谢