This is my first attempt with a GDM Theme! I basically modified the Ubuntu Default Welcome Theme (Human), using The GIMP and following the Tango Project Color Palette. I've left the logo untouched because it has its own colour guidelines. It supports resolutions up to 1600 x 1200.
1. Download the file in a handy location (e.g. ~/Desktop),
2. Go to: System > Administration > Login Window,
3. Select the 'Local' tab, then click '[+]Add...',
4. Choose the file 'HumanTangoSkyBlue-0.1.0.tar.gz' and click 'Install',
If you want to set a matching background color (it'd only be shown for a few seconds while GDM is loading), click on 'Background Color', and enter the following in 'Colour Name:' #729FCF
I hope you enjoy it! and please don't hesitate to leave your comment, either suggestions or any question regarding the installation process ;-)