There are several layers at which a GNU/Linux system's appearance can be customized. By far the most visible, especially on a multi-user machine, is the login manager screen. KDM (the KDE desktop manager) has a highly-flexible and easy-to-use XML-based theme system. If you can draw what you want, you can make it happen with a KDM theme. I'll talk you through the construction of one simple theme I designed for my ASUS Eee PC.
For years now, I have been clinging to
xdm as my display manager; years ago, I spent several days tweaking the configuration files of
xdm to get it to look "just so", and I didn't want to change it. But no more! After my last upgrade predictably broke my
xbanner configuration, I decided to spend a little time trying to get each display manager to look "right" with my original login screen design. Pleasantly, both
kdm are much easier to customize, and I had something tolerable with each in less than an hour. I found that
kdm was more flexible and gave me much closer to what I was looking for, while
gdm was quite obstinate about appearances, though it may be slightly more capable technically.