I'm creating a new ticket from the comment in ticket #59, containing screenshots of buttons having invisible text in MacOS Mojave. This is a problem in Tk itself, and it's fixed in version 8.6.8. So the fix is to download that from ActiveState. Careful, it sneakily installs it in /usr/local/bin so make sure that's ahead of /usr/bin in your path.
So ticket #59 can be closed when Kevin's fix is incorporated.
I am said commenter and I confirm the buttons are back after loading 8.6.8 as mentioned. Many thanks!
Rather than mark this as "not-fixed" (because resolution has nothing to do with TkDiff itself) we are simply saying "closed" as user has confirmed that a newer TK installation has resolved the problem