What is the best way to style the register.php file?

I have created a site using the Membership plugin. I am attempting to style the register.php file, but I have reservation about modifying the file itself since updating the plugin will overwrite my changes. Is there a better way to style the page without losing the changes whenever I update the plugin?

There is the page I am trying to style:

http://www.blueaspenphoto.com/wpmu/register/?action=registeruser&subscription=1