export QT_STYLE_OVERRIDE=gtk
[General]
font=Cantarell,16,-1,5,50,0,0,0,0,0
Right, you have to create the directory if it doesn't exist and the file in it.#2: I don't have a ~/.kde4 directory, but I'll install gtk-oxygen-engine and see what happens.
export QT_QPA_PLATFORMTHEME=qt5ct
unset QT_STYLE_OVERRIDE