$ cat /etc/X11/Xsession.d/56xubuntu-session 
# Set default environment vars in Xubuntu
if [ "$DESKTOP_SESSION" = xubuntu ]; then
    # Disable GTK 3.16 overlay scrollbars
    export GTK_OVERLAY_SCROLLING=0
    # QT5 apps to use GTK style
    export QT_STYLE_OVERRIDE=gtk
fi