Latest version of Manjaro Xfce includes kvantum and qt5ct for managing QT themes
The default ~/.profile created from /etc/skel/ at installation includes:
export QT_QPA_PLATFORMTHEME="qt5ct"
and QT5 settings (qt5ct) Style: is set to kvantum
So Xfce user should only need to change QT theme in kvantum to match Gtk theme
Dark theme is applied to not al Qt apps (have tried qt5ct and Kvantum) - #2 by bogdancovaciu