@Colin Mackenzie noted: Clearly, if you are setting up a business selling WordPress themes, your potential customers need to know that, and an obvious way to convey it is to adopt an identity that includes the words “WordPress” and “themes”.

The clear, forthright, effective - and legal - way to identify that your product is meant to be used with WordPress, is to 'just say so', without trying to use their name for your own product-name.

If you like to be a stickler (and look Cool & Professional), then include the Trademark symbol, after the protected name, like this: "We sell themes for WordPress™", or even, "We offer sophisticated WordPress™ themes for your anti-Intellectual Property website"!

Once you include the special Trademark symbol after the protected name, you can legally use the protected name, communicating to both your potential customers and to the all-important Internet search-engines.