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

@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:

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.