If you take a look at the CSS file for NestedDivFrame, you will notice it is full of sprites like the offending background one:

Code:
@sprite .content {
  gwt-image: 'background';
  overflow: visible;
  height: auto;
  width: auto;
}
But there's no ImageResource method for background in the NestedDivFrameResources interface. There is a background ImageResource in the BlueFramedPanelDivFrameResources interface:

Code:
public interface BlueFramedPanelDivFrameResources extends FramedPanelDivFrameResources, ClientBundle {

    @ImageOptions(repeatStyle = RepeatStyle.Both)
    ImageResource background(); // Needs to be in NestedDivFrameResources.

}
This means in any subclass of NestedDivFrame that doesn't use the Blue theme there's trouble.

Also, the extends ClientBundle should be in the NestedDivFrameResources interface in my opinion.