Checkout screens Marketpress does not use full width of my page

The check-out screens do not use the full width of my page? How can I change the width?

And connected to this : how can I change the lay-out of the 'review cart' screen. Specially the login and checkout directly are 'merged' in to each other and this looks ugly.

my site : http://www.norbertreijngoud.nl/store/shopping-cart/checkout-edit/