Just in case anyone else has this problem here are my notes for solving this. The problem was that all GTK applications looked really terrible, as though they were using a style from some awful early 1990’s window manager. The solution is simple though:
In KDE System Settings->Application Appearance->GTK Styles and Fonts ensure that
the GTK-Style is set to ‘oxygen-gtk’
I can’t seem to get my fonts right, despite setting the things as above. Application font sizes are way too big and don’t respond to decreasing the font size under GTK Styles and Fonts (This is a problem with menus etc. Web page and LibreWriter document content is fine, but the menus look awful.)
Nevermind. I installed the Oxygen engine (instead of oxygen-gtk or Qtcurve) and it now respons to font changes. For some reason selecting Sans Serif looks different in KDE apps and in GTK apps, but after playing around a bit I managed to get everything perfect. (Sans Serif 7.5 for KDE / Nimbus Sans 7 for GTK - for some reason all fonts now look exactly the same).