Can a GPL licensed theme be ported to a proprietary application as long as the derivative theme is placed back under the GPL?

Question

I realize that responses will fall under the "I am not a lawyer" scope, and I do intend to talk to one. But first, I wanted to get some input about the basic proposal.

If I port a GPL theme -- say a WordPress theme -- that is both free as in speech AND free as in beer to a proprietary application like ExpressionEngine, I assume that the derivative theme must also be GPL. No problem there.

However, there is concern by our management that we are in some way now required to make all of ExpressionEngine GPL -- which of course we can't since it's not our application.

I know this seems like common sense, but you'd be surprised by how little management understands the GPL (or maybe you wouldn't be surprised :). Though I did want to make sure I understood this well enough myself before proceeding.

Answer 1

I'm not quite sure how the "linking" part of the GPL works for dynamic applications.

I'm pretty sure you'd be fine not GPLing the entire application, otherwise you wouldn't be able to write GPL on any closed platform (e.g. Windows/.NET) and this definitely isn't the case.

Answer 2

As you expected: Talk to a lawyer.

Now that it's just you and me: using a GPL theme won't make the application GPL. The GPL only applies to the theme and 'derived works'.

In this case it seems fairly clear that the application is not a derived work of the theme.

Answer 3

Standard Preface: IANAL

When having GPL and proprietary code inter-operate with each other, two things must be avoided:

1. Having the proprietary code become a derivative work of the GPL code
2. Coupling the modified GPL so closely with the proprietary code that it becomes difficult or impossible to understand and/or adapt the modified GPL code without having access to the proprietary code

To be clear on 1, I think it would be sufficient that the proprietary engine have a modular themes. In other words, the GPL theme should not be "built into" the proprietary engine.

To be clear on 2, it should be sufficient that your engine have a documented, freely-available API that the theme uses to interact with the engine. If the theme is using undocumented features of the engine to do what it does, then they are too closely coupled.

Finally, if you distribute the engine and the theme together, you must be sure to distribute a copy of the GPL with the package, and abide by the terms of the GPL to include either the source of the theme or a written offer to provide its source at cost upon request.