Tista and three other dev's have done just this (see link below).

Tista has created a testing PPA to install GDM 3.2 - this is the version of the login manager which you can theme via CSS.

Warning - this is a testing PPA

At the time of writing this, there are only packages for Precise (12.04)

I originally tested this on a 11.10 virtual machine when a package existed - hence the screenshot:

I've since retested this on 12.04 - Important - see the notes below

sudo add-apt-repository ppa:tista/gdm-testing

sudo apt-get update

sudo apt-get install gdm  libgdmgreeter1 libgdmsimplegreeter1 gir1.2-gdmgreeter-1.0

_source

During the installation of GDM you will be asked which display-manager you want to run. Choose GDM and press ENTER

Reboot

changing the background

To change the background, use the AU linked question below to change the background.

The Gnome-shell stripes background can be found on GitHub.

Issues

1. Whilst there is a package for Precise (12.04), I personally haven't managed to get this to work correctly (left me at a blank screen with no login manager). Thus - it may or may not work for you - obviously it must work for the PPA developer! If you are left with a blank screen, use CTRL+ALT+F2 to log into a TTY and use this Q&A to switch back to lightDM


2. Since this is GDM - it does not appear to work with Unity. It does work with Gnome-Shell. For Unity, a blank background is seen. You have to use CTRL+ALT+T to launch a terminal and then type unity --reset to get the unity desktop to display. I'm open to suggestions why this happens and if there is a workaround...


3. If you logout and attempt to login again - Gnome-Shell very occasionally not start and you will be left with an empty desktop.

Links:

1. Discussion of implementation of GDM 3.2


2. How to set gtk-style and background in GDM3