"The Game Genie is useless by itself, it can only enhance, and cannot duplicate or recaste, [sic] a Nintendo game’s output. It does not contain or produce a Nintendo game’s output in some concrete or permanent form, nor does it supplant demand for Nintendo game cartridges. Such innovations rarely will constitute infringing derivative works under the Copyright Act."
So, if the theme doesn't use actual Wordpress code (IE: beyond function calls) or "supplant demand" for Wordpress itself, it's not a derivative work.
Would you consider a Nintendo cartridge a derivative work of the Actual System? It obviously uses components of the actual system.
This court case: http://scholar.google.com/scholar_case?case=1086785624507896...
Galoob VS Nintendo
This was about the game genie.
Here is what the court concluded:
"The Game Genie is useless by itself, it can only enhance, and cannot duplicate or recaste, [sic] a Nintendo game’s output. It does not contain or produce a Nintendo game’s output in some concrete or permanent form, nor does it supplant demand for Nintendo game cartridges. Such innovations rarely will constitute infringing derivative works under the Copyright Act."
So, if the theme doesn't use actual Wordpress code (IE: beyond function calls) or "supplant demand" for Wordpress itself, it's not a derivative work.