Themes and plugins cannot be talked of as derivative works in the legal sense I think.
But the FSF do say that if the module cannot exist apart from program in this case Wordpress then the whole is to be considered the program. Combining non-GPL software with GPL is also a somewhat gray area it seems. You cannot however write a wrapper function around GPL and make a bridge to non-GPL. Then the whole will be considered GPL.

Also the only exception to modules not affected by GPL is if they are initated using fork and exec. If Wordpress only calls the constructor of a theme/plugin class then that is a grey area. But this is however not the case with either themes or plugins.

If the theme is useless without the CSS or images one could easily argue that those files are also affected by the GPL. I would say that you cannot make themes or plugins to wordpress that uses wordpress functions or is dependant upon wordpress to function and not have the theme/plugin become GPL.
That is the whole point of the GPL and why you really should use LGPL for your software.
Which you can see if you look at the following perspective.
If you integrate stuff into the TinyMCE editor that is a whole different area since the TinyMCE has a different license than Wordpress (LGPL) you can put a non-GPL licence on the code that TinyMCE interacts with and a GPL licence with a exception on the code that interacts with both Wordpress and the TinyMCE.

But in essence if a theme or plugin calls methods in Wordpress then those files are affected by the GPL.