It is possible to use custom CSS and HTML to completely customize the look of the header in your instance.
For some Service-now.com implementations, there may be a need to modify the default login page. In other implementations, it may be necessary or desirable to remove the login page altogether (this scenario would only apply if you have set up Single sign-on for your instance and you didn't want users to authenticate directly against Service-now). Whatever the reason is, you may find it necessary to modify the behavior of the Service-now login page. If you do, there are a couple of options I would recommend.