This plugin will let you change the css of your login and register pages.
This is a full featured plugin to help you edit your login just the way you want it. Lets you use or block the original wordpress css (the first plugin to do this). Comes with one pre made style you can use or deactivate. Very easy to create your own style using the style tags givin in the plugin. Please visit http://vasthtml.com/js/login-xchange-plugin/ for usage instructions.