What do you mean? Themes for mobile web pages, or overall themes for mobiles?
In either case, I doubt that you are going to find much in the way of specific tools, at least not generically for "mobile". There are too many differences between different "mobiles".
For the first, I'd think you'd use the same tools you use for creating any website theme - except you are going to make it smaller - and in multiple versions.
The latter isn't even possible for many mobiles, as they are limited to built-in themes. For others, the carrier has locked them so that themes can only be downloaded from the carrier. For ones that can actually install a theme and it's possible to freely download them, I'd guess there must be some site where you can go and punch in your model # (if it's popular) and then design a theme - but I'm unaware of any myself.