Feature request: Make GDM background color and image configurable with gsettings.

Customization of the greeter is a frequent requirement from corporate customers.

Currently the background (plain color or image) of the default greeter is hard coded in the style embedded in the theme of gnome-shell. Customizing this background requires to extract the style sheet from gnome-shell's gresource file, modify it manually, recompile and deploy the new gresource file, and optionally, depending on the distribution, add an alternative.

For the end user (typically sysdamins) it would be much safer and easier to be able to modify the settings through gsettings as they usually do for other applications and in particular for the background of desktop.

It would make sense to add such keys to the schema of GDM (org.gnome.login-screen) and read it from gnome-shell. Then apply it as an inline style to override the default theme style.

As a PoC you'll find attached what we would like to do in Ubuntu. Of course, if the implementation upstream is similar to this proposal, the namespace of the schema would be changed to GDM's one.

gnome-shell_3.38.3-3ubuntu1_3.38.3-3ubuntu2.patch

Thanks for considering this proposal.