How do I style the Register page if I'm using the 'Fixer' theme?

Hello
I'm using the Fixer theme which works very well.
The only problem is when a user clicks 'Register' as the page is then completely un-styled- without any theme.

See below-

Fixer is fine on http://rovello.co.uk but doesn't work on http://rovello.co.uk/wp-signup.php

How do I make Fixer the default for new registrations too?

Duncan