I have found an even better solution (for me, at least), which is disabling CSD (Client Side Decorations) altogether by simply installing:
https://aur.archlinux.org/packages/gtk3-nocsd-git/
Additionally, open xfce4-settings-editor
and in xsettings
set Gtk>DIalogUseHeader as false (untick box), to make sure the file picker is brought back to its former glory.
I expanded on this topic in this tutorial I have made → Tutorial: Restituting consistent window theming on XFCE - aka revert new window borders (CSD) to previous mode (SSD)