Credit for this tutorial goes to Webupd8. I am simply reposting the information here. The original tutorial can be found at http://www.webupd8.org/2012/05/install-mint-display-manager-mdm-in.html. It was written for Ubuntu users, so I have removed the part that details how to install MDM since it comes installed with Mint 13. I have tested this out with an old Star Trek GDM theme I used to use, and it works perfectly. Thank you Clem for bringing back the old GDM goodness with MDM. I think this is a great move for Mint!
From the article:
Linux Mint 13 has introduced a new display manager called MDM, that supports themes just like the old GDM 2.20 (which has been used as the base for MDM).
You can download more MDM themes from Gnome Look. To use a theme, you'll have to extract the downloaded archives because you'll have to make some changes:
rename the "GdmGreeterTheme.desktop" file to "MdmGreeterTheme.desktop"
open "MdmGreeterTheme.desktop" with a text editor and change "[GdmGreeterTheme]" to "[MdmGreeterTheme]"
Once you're done, archive the theme folder to .tar.gz and use the MDM Configuration Tool (mdmsetup) to install the new theme:
Note: when renaming the .desktop file, it may automatically be renamed to "MdmGreeterTheme.desktop.desktop" - if that happens, use the terminal to rename it to "MdmGreeterTheme.desktop" (a single .desktop) using the "mv" command.