You should be able to change the theme and the icon theme from the screenshot here: https://www.gimp-forum.net/Thread-Gimp-2...01#pid7901 using the relative positions in the preferences list. That shows the icons. The themes are just above it and the last in the theme list is 'system' (it will be gibberish to you) try that one first.
The not simple bit.
The font its self is more problematic. I think this has been reported before for OSX, but I am not sure. A search came up with this similar state for Windows. https://gitlab.gnome.org/GNOME/gimp/issues/2826 You might want to report the problem as a new issue.
Alternative. Try the Gimp 2.10.12 8 version from Partha http://www.partha.com It might be better.
The not simple bit.
The font its self is more problematic. I think this has been reported before for OSX, but I am not sure. A search came up with this similar state for Windows. https://gitlab.gnome.org/GNOME/gimp/issues/2826 You might want to report the problem as a new issue.
Alternative. Try the Gimp 2.10.