QT is not cute in ubuntu 18.04 ! See below screenshots. I have the same issue with scribus and vlc.
I tried deleting all config files that I know about, no change. I tried both qt4-config and qt5ct, without success (changes made in fonts tab of these utilities have absolutely no effect).
What else can I try ?
QT_QPA_PLATFORMTHEME
set to? I think it needs to be set toqt5ct
for it to work.