Lets assume for a moment that WordPress themes are strictly GPL.

Tell me why I couldn't sell a stand alone HTML/CSS/images package (with restrictive license) which contains CSS & images that just happen to be compatible with a bare bones GPL theme. You could instruct a buyer that "if" they wanted to use the HTML/CSS/images package with the free GPL theme, they simply discard the HTML portion of the package and either upload the CSS & images as a child theme or overwrite the existing CSS & image files of the GPL theme.

I would think that since the HTML/CSS/images package is not dependent on WordPress, there is no way it would be forced to fall under the GPL. I would also think that the optional act of using parts of that package with a bare bones GPL theme would not magically make the new creation GPL as the two could operate without each other. The end user would not be able to freely distribute the combined files as each carry a different license.

Another example: If I bought a HTML only template from Template Monster and caused its CSS and images to work on a GPL theme for a client, I highly doubt that client would be free to distribute that final product as a GPL theme since the Template Monster files have a restrictive license.

Some may argue this is not within the "spirit" of the GPL. I'm simply trying to discover what is "legal" within the GPL.