Short answer: As root:
Code:
xhost +SI:localuser:gdm
sudo -u gdm dbus-launch gnome-control-center
Then select Appearance and Background.
Tried on Centos version 6.5 GDM version 2.30.4
I had done a bunch of Google searches and failed to figure this out. I saw lots of places others asked the same question and got answers that I could not understand. So once I figured it out, I'm posting this to try to help someone else.
The
key concept that I didn't think through (or I would have gotten this much more easily) is that the login runs as localuser:gdm and its background is just that user's desktop background. (I had expected it to be some special setting because the desktop background was different from the login background).
So all the places where the answer to setting the GDM login background was given as a description of setting the desktop background were not (as I thought when reading them) misunderstanding the question. They were just skipping the key concept needed to use the answer (that, of course, you need to do this for the user gdm not for your own user).
Details:
Code:
xhost +SI:localuser:gdm
so far as I understand, tells your currently running X-server to accept a GUI opened by that user (sorry I'm off on terminology and details).
Code:
sudo -u gdm dbus-launch gnome-control-center
Runs gnome-control-center as user gdm. I haven't looked up what the dbus-launch part does. I had to su to root before sudo to gdm. Those who understand and/or configure sudo better might be able to do that more directly.
I think the results of doing all that are stored in hidden (starting with .) directories in ~gdm/
I may take time later to try to figure out which files in there control the things I care about, since I prefer to understand text config files, not just the config programs.
