@Andreas Nurbo -

If the theme is useless without the CSS or images one could easily argue that those files are also affected by the GPL.

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".

Further, those CSS and image files can be plucked out, and replaced with entirely different CSS and image files - meaning that, by definition, the theme is not dependent upon those specific, original CSS files.

Second, your argument becomes incredibly twisted, and untenable.

To wit: if a theme is dependent upon a CSS file (or an image file) that has its own (for the sake of argument, non-GPL) license, as in, that theme is "useless" without said file, then why would the file upon which the theme is dependent somehow inherit the GPL?

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.

Do you really want to try to make that argument?