No - it is my bad. I spent the best part of yesterday evening scratching my head as to why it wasn't working.
The issue is that the package startup script is diverting GTK2 to look at a different location and didn't include this well-known location as well (in other words it is a bug!!).
Where GTK is looking is inside the program package itself under Contents/Resources/etc/gtk-2.0/ for a file called gtkrc. That will need to be fixed, but for now I have found a way to bypass it.
The bypass I did was as follows - I went into Finder to Applications and Right-clicked on xtrkcad to "Show Package Contents" then I followed Contents/Resources/etc/gtk-2.0 . I pasted the file in that directory and renamed it as "gtkrc" (note - NOT gtkrc-2.0 and with NO dot at the front).
Once I did that, the font settings took. The actual content I used was
class "GtkWidget" style "default-style"
You can play with the "12" to get a size you like that also fits - as some places fonts go have a formatting issue if it is too large.
Sorry about that - I