Possible Duplicate:
How can I change the login screen theme?
How do I change how the login screen looks in Ubuntu 11.04? I want to change both the background and the design of the grey rectangle (where the login form is) in the center. I've found some sites explaining how do do it for 10.04, but only for changing the background image.
You can use a tool called GDM Tweaker:
GDM Tweaker is a very simple application that allows tweaking the GDM login screen look: change the GDM background image, hide the username list, enable Compiz and change the GDM login screen icon or GTK theme.
To install GDM Tweaker in Ubuntu 11.04:
sudo add-apt-repository ppa:madnessmike/madnessmike
sudo apt-get update
sudo apt-get install gdm-tweaker
Note: To be able to change the GDM Background / Theme you must place the images (must be only .png or .jpg) into /usr/share/backgrounds/ and the Themes into /usr/share/themes/ for GTK themes and /usr/share/icons/ for the icons.
To be able to copy the themes / backgrounds to these folders, open Nautilus as root using gksu nautilus .
Source : WebUpd8.
