I tried pretty much everything that was suggested. No joy. (Adding to the
fun is no easy way I know of to tell whether a particular application is
GTK2 or GTK3, making it possible to break some things while not fixing
others.)
Anyway, after the various gnome and gtk tweakers failed to give me what I
desired, I happened upon, in Tmenu > Settings > Settings > KDE System
Settings, which is apparently a KDE-heaven-knows-what-version
configurator, something called "GNOME Application Style (GTK)" and there
I was able to change the typeface and size and all became wonderful.
(Could change themes, too, if I wanted.) I'd love to give a better
description of the configurator, but it has no "About" or "Help" or
anything. Anyway, it works. (In top is shows up as systemsettings5, so I
suppose it is a KDE5 configurator. I have no idea why I have it.)
--
dep
Some pictures:
http://www.ipernity.com/doc/depscribe/album