Ubuntu, like other Linux distributions uses the GNOME Display Manager (GDM) as its, well, display manager, though most of us tend to think of or refer to it as the login screen. To customise the Ubuntu login screen, such as setting a different theme or icon set, changing the login screen background image or colour, or getting night light to apply when viewing it, you can use a 3rd-party tool, Read More
From OMG Ubuntu