@markvanlan recently did some work which might be useful for you
https://github.com/discourse/discourse/commit/f07c4a781c6c054e5d2dc05ab6c0fd0b6cbb0645
There is some usage info in the comments here:
https://github.com/discourse/discourse/blob/f07c4a781c6c054e5d2dc05ab6c0fd0b6cbb0645/lib/tasks/themes.rake#L5-L26