Your comments

I'm afraid it would clutter both the use case for easybacklog and, more importantly, the interface/implementation.
This is an important consideration.  We've found themes in easybacklog to be mostly unusuable as envisioned exactly because of this.  A backlog should have exactly one ranked order, but themes make a mess of that.

To that end, how we HAVE found themes useful is to think of them more like milestones - essentially units of time.  Since story priority must equate to time, then it's natural to move stories in and out of themes based on units of time.  

You could use themes using this idea following any logical grouping of time you felt was appropriate, e.g. M1, M2, M3 or alpha, beta, production, or Q1, Q2, Q3 etc.