the file you need to edit is admin/wp-admin.css
however since changing that does change the entire admin area, you might want to do what I did.. change the style sheet definition within wp-login.php/wp-register.php/etc.. to another .css and use the original wp-admin.css as your basis for that new new stylesheet (since the classses and whatnot are already outlined there for you).
hope thats less confusing than it sounds like to me reading it. It really wasnt all that hard to implement. my registration/login pages match my site quite nicely and it was worth the effort :)