Thanks. I had started diffing the old vs new dirs and it looked like it was going to take some fiddling (exactly as you noted). Was hoping I was wrong and there was something more straightforward.
Perhaps there is a more "official" way to add a new theme that won't break when updating. Or maybe...
Grabbed latest release, thanks!
The exe did not work for me -- first it got blocked by MalwareBytes, then after I disabled MB it grabbed an HTML file instead of the BAT file it was trying to grab (which obviously didn't run).
Was able to just paste over the top of my existing MediCat USB stick...