Package ubuntu-mate-default-settings ships /etc/X11/Xsession.d/99mate-environment which contains (among other things):
# QT apps to use GTK styling export QT_QPA_PLATFORMTHEME=gtk2
Package ubuntu- mate-default- settings ships /etc/X11/ Xsession. d/99mate- environment which contains (among other things):
# QT apps to use GTK styling PLATFORMTHEME= gtk2
export QT_QPA_