before, I used the Loginized software who do the job. But now with Gnome 3.36, this software seems to be obsolete (see that information in the Loginized github).
As from Gnome 3.36 onwards (https://help.gnome.org/misc/release-notes/3.36/) this app might have become obsolete and most likely will not function the way expected due the major changes they introduced with the new release. So be cautious when using the app.
So I have to find another way (probably manual) to change the default background and also the default gtk-theme of the login screen. But I don't know how to do it ? Must I use dconf editor ? Must I modify a file but which one ?
If you have an idea on that question, I'll take it.