Many thanks for the help. This is what I did now and it works for the Menu Bar.
I edited the startup file below (my Xtrkcad install is under /Applications/Trains)
Just to keep the rc file in my macos home directory rather than digging down into the install package...
Also did this but not really needed if one just places the gtkrc file in $HOME. This way one can edit either file gtkrc or .gtkrc-2.0. One is visible and the other hidden.
ln -s .gtkrc-2.0 gtkrc
ls -la .gtkrc-2.0 gtkrc
-rw-r--r-- 1 zeeb staff 212 17 Jan 18:09 .gtkrc-2.0
lrwxr-xr-x 1 zeeb staff 10 18 Jan 14:19 gtkrc -> .gtkrc-2.0
Now, any suggestions for increasing the text size on the Hot Bar?