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.)