How to apply CSS to a DialogBox using ClientBundle?

I'm trying to apply some CSS to my subclass of DialogBox via ClientBundle. When inspecting the DOM I can see an obfuscated class-name got added (class="gwt-DialogBox GF2AVPFBPD") but it does not bear/apply any propertes/changes...

class RegistrationDialog 
  extends DialogBox 
{
  interface MyClientBundle 
    extends ClientBundle 
  {
    @Source("regDlg.css")
    MyCssResource css();
  }

  interface MyCssResource 
    extends CssResource 
  {
    String registrationDialog();
  }

  MyClientBundle myClientBundle = GWT.create(MyClientBundle.class);

  RegistrationDialog()
  {
    add(uiBinder.createAndBindUi(this));
    addStyleName(myClientBundle.css().registrationDialog());
  }
  ...

regDlg.css:

.registrationDialog
{
  background-image: url("logo.png") !important;
 开发者_StackOverflow中文版 background-color: red !important;
  color: red !important;
  font-size: xx-large !important;
}

What am I missing please?

You're forgetting to call CssResource.ensureInjected(). From the documentation;

At runtime, call CssResource.ensureInjected() to inject the contents of the stylesheet.
- This method is safe to call multiple times, as subsequent invocations will be a no-op.
- The recommended pattern is to call ensureInjected() in the static initializer of your various widget types

Check out the answer on this question for more code info.