The login form design is not a tempalte.
The login form design is controlled by css.
to make changes to the login form design, you need to edit the css.
The Firebug extension in Fire Fox is helpful for examining page elements and finding the css that controls them to determine where edits would need to be made:
as for replacing the login form with your own, you could do this, but make sure that the form action, and the names of the form elements are the same as the original.