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