Changing the GNOME App Theme outside of GNOME

[Jonas Damm <emails jonas-damm de>]

Hi,
I am using a windowmanager but also have installed GNOME on my system.
I want to change the system theme on the fly with shell commands. For
that i use xfsettingsd to switch from "Adwaita" to "Adwaita-dark" and
vice versa.
This works for Firefox, LibreOffice and other Applications, but does not
seem to effect GNOME Apps like Evince.

How can I change the GTK Theme for all my GNOME Apps without running
GNOME?

Thanks,
Jonas