- Add the css to assets

backend/assets/AppAsset.php

@@ -19,6 +19,7 @@ class AppAsset extends AssetBundle
     public $baseUrl = '@web';
     public $css = [
         'plugins/bootstrap-datepicker/css/datepicker3.css',
+        'plugins/bootstrap-select/bootstrap-select.css',
         'plugins/bootstrap-datetimepicker/css/datetimepicker.css',
         'plugins/switchery/switchery.min.css',
         'plugins/powerange/powerange.min.css',