What license can I apply for a theme I sell?
One technique you might consider is committing your basic theme to a project on drupal.org, distributed under the GPL.  Then, develop premium css and graphics that can be installed over your basic theme using instructions that you provide. The later package could be distributed under a license of your choice.

It is okay to use a separate license for the css and graphics files because they can be considered separate works that stand on their own. To make it more clear that the css and graphics are separate works, you could distribute them with a static html file that shows what the final theme should look like. Your customer has the right under the GPL to combine the css and graphics with his GPL theme; tt would be a violation of the GPL to distribute the combined works, but they can be used by the licensee of the separate works (your theme).

Finally, regarding the "basic theme", note that there is nothing special about css and graphics vis-a-vis the GPL; it is still necessary to deliver the "source" of your works along with the "binaries" when they are included in a GPL-licensed distribution. If you built the css using some tool, then you must deliver the source files that were used to produce that css. If you created the graphics in photoshop with layered psd files, then you must also distribute those files with your theme, and so on.

Edit Edit Edit: Reworked my prior answer(s) for clarity, and to remove a mistake.

Postscript: I believe that the FAQ referenced below is partially incorrect; graphics and css are not excluded from the GPL because they are "data"; if they are an integral part of the work being distributed, then they are covered by the GPL as much as the code is.