This post explained how to do exactly what I was wanting: UI-Themer missing from OpenRepos
It worked for 4.0.1.48. I used nano editor to edit the silica-configs.txt file, as root file explorers don’t seem to work with the 4.0 update.
I tried this on an Xperia 10, changing the ratio to 1.5 - also tried on an XA2 and it worked great - but what I enjoyed afterward was being able to change text size in the settings to something that was more reasonable - normal was small, and large was normal when running at a lower theme pixel ratio.