If you check the wp-register.php file, it calls "wp-admin/wp-admin.css"
So you could always style another page specifically for registering and have the wp-register.php call that instead.
You'd need to check the login.php too - 2 sep files = 2 sep functions I'd guess. And you could always have additional information on one ?
Keep a backup of the original files though - just in case ;)