Ubuntu 9.10′s new login screen looks great — brown, black and white. Much improved over the previous version.
However, if you try out the Accessibility options, in particular, the “improve contrast” option, it goes ugly, with a nasty grey bar.
The problem is that, even after you deselect the option, the theme does not revert back, even after restarting again. The problem is that some needed theme files are deleted.
The solution is simple — you need to re-install gdm. Either fire up Synaptic, search for gdm, and then mark it for re-installation, or do
sudo aptitude reinstall gdm in a terminal.
There you go — sexy login screen back again.