Just as a test to check possibility. Unzip the attached Gimp theme, it unpacks to a folder 3-samj.
Put the 3-samj folder in your Gimp user profile, Themes folder. Some info on where it might be for MacOS here:
https://www.gimp.org/tutorials/GIMPProfile/In Gimp -> Preferences -> Themes select that theme and re-start. If you get a larger font, then modifying gtkrc should work (and MareroQ plugin maybe is writing gtkrc to the wrong place for your MacOS installation).
If it does not work, blame Apple for screwing things up