In dom0:
$ sudo qubes-dom0-update qt5ct qt5-qtstyleplugins
Then, run:
$ export QT_QPA_PLATFORMTHEME=qt5ct
Then:
$ qt5ct
Here you select Style: gtk2
(to match your current XFCE settings) and you can select also Fonts
and Icon Theme
.
Only QT_QPA_PLATFORMTHEME=qt5ct
needs to be made persistent.