We researched this case and we consider that theming RadCarousel in Visual Style Builder functionality is not fully relevant. This consideration is based on the following facts that: 
- RadCarousel rotates images (which cannot be set through Visual Style Builder) and should be set programmatically per item
- There are only two navigation buttons which use the default theme for RadButton, so no additional theming is needed here
- The background is the only relevant thing that can be themed, but we consider it would be rather overhead than something useful for our users.