D'oh. Stupid copy&paste error. Fixed on master. You might want to try a recent incremental build:
https://repo.jenkins-ci.org/incrementals/org/jenkins-ci/plugins/simple-theme-plugin/0.6-rc72.a126264b6312/
Woohoo! .6-rc72.a126264b6312/ works for me now. caveat: ours was a pretty simple theme. Validation against a more sophisticated use case may be beneficial.