I just uploaded version 1.3.0 (for KT 5.1.0+ ONLY) to
http://ge.tt/3ZRY9XE/v/01
I'm not a 100 percent sure about everything, but it should at least *run* on 5.1.0+ Kindles. That said, it should work at all, but I can't tell if it really works correctly. (in my particular case, overriding the "normal" (serif) font did not have any effect.
I need some other testers for this, so please report back here.
UPDATE: That means: please test-drive as much as you can. Change all font styles, look at where you expect changes to appear, and whether they work or not. Ideally, check all of the four different font substitutions (serif/sans serif/monospace/condensed) and report back about what works and what doesn't.