When you customized your Ubuntu desktop to replace the brown theme, you may have overlooked the login window. GNOME’s login window, called GDM (GNOME Display Manager), can be easily customized by installing themes.
On the theme’s page, click the link to download it and save the file to your desktop.
Open System->Administration->Login Window. Select the Local tab. Now drag and drop the theme file from your desktop into the theme list.
Choose the theme you want to use by selecting the option button corresponding to the theme. It’s even possible to use random themes by selecting Random from selected in the Theme drop down box and selecting multiple themes.
You can delete the archive file you downloaded once the theme is installed.
Side note: I seem to have found a bug: Themes in .tar.bz2 format can be installed by dragging them into the list, but only .tar.gz files are listed when using the Add button. Anybody know what’s up with that?