Looks like Envato (hi @natman) does the following, a split license for themes:
Envato Market’s license for themes or plugins sold on the Envato Market sites covers all the components of these items, except for the specific components covered by the GPL. This is why it’s called a split license: because different license terms can cover individual components that make up a single item.
The PHP component and integrated HTML are covered by the GPL. The rest of the components created by the author (such as the CSS, images, graphics, design, photos, etc) are covered by the Envato Market license.
That would only cover themes, though, where there are a lot of non-code assets.