I don’t see why this should be a problem (assuming, obviously, that you have the copyright for the theme you’re selling). 
While Discourse is GPL-licensed which requires derivative works to also be GPL-licensed, I’d say that considering a theme to be a derivative work is pretty far-fetched – most likely, you’re only shipping CSS code and your own assets anyway.
If you want to be 100 % safe, wait for a reply from someone from Civilized Discourse Construction Kit, Inc. on this subject.
I different case could probably be made if your theme included significant portions of (modified) Discourse source code, e.g. handlebars templates.