Customizing plymouth login screen background, fonts and gtk...

1) Open terminal, copy and paste the following command.

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

2) Close terminal and log out. Once at login screen you'll see an Appearance pop up. Make your changes and log in.

3) Now to disable Appearance pop up from login screen. Open terminal, copy and paste the following command.

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

Note :
Tested on Ubuntu 10.04 and Linux Mint 10.