WP customization questions

I am trying to customize our WP MU members section, so I am wondering how I can customize

A. wp-signup.php page. Here I have our own (html) signup page with its Header/ Footer / Content which I need to implement as the signup page for our website.
Problem here is that all tutorials I find are talking about changing background color, may be playing a little with the CSS values, but none I found was talking about how to fully customize this page including (header/ footer/ content)...so can someone please tell me how I can do that? OR if anyone know a good tutorial that can explain how to do that?

B. wp-login.php page. Here I also have our own (html) login page with its Header/ Footer / Content which I need to implement as the login page for our website.
Here I am facing the same problem as above so again if someone can please tell me how I can do that? OR if anyone know a good tutorial that can explain how to do that?

Thanks in advance for your help and time