GTK setting doesn't seem to work
Under "GTK Applications", the widget style is set to QTCurve. The fonts are set to "Use my KDE fonts in GTK+ applications", although there's no choice there; the other alternative of a specific font is greyed out.
I'm beginning to think there might be a bug here that prevents the application of KDE fonts to GTK applications, since a similar problem has shown up elsewhere. In the past, the KDE fonts did seem to be used in Synaptic and such, but I don't remember how long ago that was. I noticed that QTCurve wasn't installed, but installing it didn't make any difference.