brent the ignorance of this app was all mine
It's the first time I've ever seen it. I've tried before, but it was whie I was signed into the terminal as root, doing a bunch of maintenance. As root, all I could get were failed assertion errors. I finally tried it as a regular user, and suddenly things worked.
Based on that, I now assume that themes are installed per user profile, not per system. As the only user of my machines, I didn't immediately appreciate that difference.