Custom signup page TO EACH THEME in WPMU

I'm wordpress network (wpmu) marketpress owner / developer.

When my costumers do your first buy at example01.maindomain.com, they need to register, but when I offer register link ( using wp_register() ) , they are redirected to maindomain.com/signup.php.

I need a custom signup page TO EACH THEME, once each subdomain will use your own theme and the buyers don't need to know who is "maindomain.com"

Who can help me, please? Thank you!