@Andreas Nurbo -

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.

Except that:

1) HTML is output by the PHP, and is otherwise irrelevant

2) Each PHP file is a separate work. Each image is a separate work. Each CSS file is a separate work.

(Again, the truth of this statement is understood when one realizes that any individual PHP file, image, or CSS file can be replaced by another such file - especially since the replacement file can have an entirely different license than the original.)

When this point is understood, the argument that the theme is only considered to be a work "as a whole" falls apart entirely. True, a theme as a whole is a work itself; however, that doesn't negate the individual works from which the theme as a whole is constructed.

Each individual work doesn't necessarily require WordPress in order to function - primarily, the images and CSS.

(To take the point even further: technically speaking, one could write entirely different functions with the same names as the WP functions, thus allowing even a PHP file to function outside of WordPress.)

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)

1) The GPL inheritance only applies to those components of the theme that make calls to WP-defined functions. Since neither images or CSS files make calls to anything - in fact, they do nothing except exist - GPL inheritance doesn't apply.

2) The theme isn't only considered as a whole; never has been, never will be. One could take components from one theme and combine them with components of another theme. One could take components from a theme and use them elsewhere. One could take components from elsewhere and use them in a theme.

3) As for the theme-considered-as-a-whole argument: such argument applies to the rendered result - which applies not to the theme components, but rather to the theme design. WordPress does not own the copyright to a theme's design, but rather the theme designer owns that copyright. Since WordPress doesn't own the copyright, it can't impose its license on the work - that is, the design.

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.

No, that analogy doesn't work, because a plugin isn't as analogous as a library. A library is designed to work with many (or any) programs, so long as the program calls it properly. A plugin is designed to work with a specific program.

An image (or CSS) file can work with any PHP/HTML file, provided that the PHP/HTML file calls it properly. Such file does not depend on WordPress, at all.

Even to mix the analogies, and consider a plugin that calls a library: the non-GPL library called by the GPL theme does not inherit GPL from the plugin that calls it.

Again, do you really want to try to make the argument otherwise?