QT5 APPLICATIONS STYLING IN XFCE ENVIRONMENT

QT5CT

$ sudo apt install -y qt5ct qt5-style-plugins adwaita-qt

ENV

Modify /etc/X11/Xsession.d/56xubuntu-session : 

QT_QPA_PLATFORMTHEME=qt5ct
QT_STYLE_OVERRIDE=adwaita

ORACLE VIRTUAL BOX

Dans le menu application de XFCE, remplacer la commande de lancement par : VirtualBox -style Fusion %U