Ultimate Facebook Registration Page

Hello.

I am trying to customize the forced FB registration page.

If i tick the Force Facebook registration button, then upon clicking on Register button it takes me to:

http://tranceil.fm/new/wp-login.php?action=register

but as you can see its stretched allover the main page.

How can i customize it so it will show the registration box on a different page? (a customized one..lets call it 'custom.php')

Thank you for your help.