GDM (GNOME Display Manager) is a application that manages your graphical user logins and display servers in your Linux system. Ubuntu also comes with GDM. Now you can change all GDM setting easily with Login Manager Settings application. It is written in python and released under GNU Affero General Public License v3.0.
Using this app you can change login screen background, shell, icon, cursor, fonts, Top Bar clock and power settings, sound, mouse and touch pad setting, display, logo, welcome message and more.