KH-Lite. The background for KH-Dark looks good to me! More subtle, easier on the eyes Dream Eater also looks really nice, smooth & professional. I would suggest a consistent logo throughout all of the themes, just for consistency/reliability
I don't really think that having a consistent logo is necessarily a good idea on a forum like this. If it's for a business, then yes, consistency would be the best way to go. However, for a fan-based forum, I think users like seeing the different graphics instead of a single image.
For certain themes that coincide together, or dual themes, like the 2.0, then it's fine to have a single logo for both. But when you have a large variety of themes, making a new theme specifically for said theme would be more pleasant to the eyes. In some cases, the logo you would use for consistency wouldn't fit well with the theme you're using.