Ubuntu Guide shows you how to change the Ubuntu 11.04 Login Screen Appearance

You may be not satisfied with your Ubuntu 11.04 Natty default login screen appearance. You can change it using Ubuntu-Tweak, but it only changes the login screen logo and the background and sometimes Ubuntu-Tweak even doesnt work. Heres a way to launch Appearance Preferences window while at Ubuntu 11.04 login screen,

  An effective way changing Ubuntu 11.04 Login Screen Appearance