Since it says “Noncommercial — You may not use this work for commercial purposes.” I wouldn’t do it only because you would be using it for commercial purposes to advertise your theme even if it is a demo. I would contact the owner of what you’re trying to use and get clarification that it’s alright to do.
I think you couldn’t use it since the license stated that:
Noncommercial — You may not use this work for commercial purposes.
The demo is also a part of your commercial work.