Make the background of login and lock screen configurable
I saw several requests in upstream/downstream which want to customize the grey background of login and lock screen. Though a new design for login & lock screen has been proposed during 2017 GNOME Shell UX Design Hackfest, it seems there is not implementation work yet. So I think it's still a reasonable request.
And I found:
- the approach by Ubuntu community was rejected.
- the approach by Mageia community doesn't migrate to gitlab, the bug status is still New
I wrote a patch to openSUSE and my solution is similar to Mageia's, I will submit the commits later.