How can I change the login screen theme in GDM?

Use this command in the terminal:

sudo cp /usr/share/applications/gnome-appearance-properties.desktop /usr/share/gdm/autostart/LoginWindow

Logout, and you will be prompted with the Appearance window. change the theme as you change it for the desktop, and login back, and type this command:

sudo rm /usr/share/gdm/autostart/LoginWindow/gnome-appearance-properties.desktop

That's it.

This depends on what you want to do with the login screen. You can configure some basic options in the Login Screen Settings app (System/Administration/Login Screen) - whether a user should be automatically logged in, the default session etc.

However to change the login screen more graphically Ubuntu Tweak can be used to change the background, logo etc.

U can use GDMtweak to change theme and Icon. see the link http://www.webupd8.org/2011/05/change-gdm-theme-background-in-ubuntu.html