James Dimick, author of the Easy Admin Color Schemes plugin for WordPress has released a color scheme based exclusively on WordPress 2.3. Now, when you install this plugin, the default color scheme selected is called “For the Love of 2.3” which will take the new WordPress and make it look like the old. Something many WordPressers have wanted since the release of 2.5. However, James mentions that:
It isn’t a completely faithful recreation but I think it’s pretty darn close.
The scheme was created purely from CSS with no modifications to the core code. So while you might not be able to make WordPress 2.5 look similar to the way WordPress 2.3 did in terms of the user interface, you can at least slap on old faithful for that nostalgic look.