@Chip Bennett -
Should first clarify I did not intend "become" as in GPL spreads against the authors will rather to be compatible the theme as whole have to be GPL inorder to function with GPL software. Replace become with "be released as".
First, define “useless”. A theme will still render just fine (as in, without errors) without either the CSS file(s) or image file(s). For a plethora of evidence, see “CSS Naked Day”.
Of course it can render I'm not an idiot but you are missing the point. In the theme case the theme will be useless as in not providing the intended function (in almost all cases) when you remove CSS and images. The combination of html,php, images and css is what constitutes the theme or the work, this work then needs to be combined with Wordpress to function. If you remove the images and the css it is no longer the same theme/work.
If the theme makes use of calls to Wordpress functions which are GPL the theme has to be GPL or some other compatible license. That is why I would interpret the images and the css also has to be GPL. As the theme is to considered as a whole and would not be the same theme if you removed the CSS and images. You could also write an exception with the GPL saying it is allowed to use these non-GPL stuff given certain conditions. (See last link)
If I add a module to a GPL-covered program, do I have to use the GPL as the license for my module?

I'd like to modify GPL-covered programs and link them with the portability libraries from Money Guzzler Inc. I cannot distribute the source code for these libraries, so any user who wanted to change these versions would have to obtained those libraries separately. Why doesn't the GPL permit this?
If a program released under the GPL uses plug-ins, what are the requirements for the licenses of a plug-in?
But this might be even more applicable in some cases
What license should I use for website maintenance system templates?

To make an analogy to a compiled program: your argument is analogous to saying that, if a compiled program dynamically links to two libraries - one GPL and the other not - and if that program is dependent upon both libraries to function, then the non-GPL library must inherit the GPL from the GPL library.
First the software would not be compatible with the GPL as is so there is no inheritance. Second your analogy is flawed. It would rather be GPL software uses a GPL plugin that uses a non-GPL plugin.
You have to write an exception in the GPL license but the theme/plugin would still not be proprietary but GPL. The problem is also who should write the exception. The theme/plugin developer or Wordpress copyright holders since the theme/plugin is an extension of Wordpress and not a separate entity.
What legal issues come up if I use GPL-incompatible libraries with GPL software?