Second, I still fail to see how you can argue that themes are derivative works. A derivative work, as defined by Wikipedia is "an expressive creation that includes major, copyright-protected elements of an original, previously created first work." A theme starts out as an image in Photoshop, and then is built up with CSS and HTML, and then lastly has a few function calls from WordPress added in. The vast majority of the end product is purely original. It's like arguing that a software package like Firefox, GIMP, or Blender must be licensed under the GPL because it runs on the Linux kernel, which is licensed under the GPL. An application may run on Linux, but the majority of the code is its own.