Hi Axel,
When using any page that sits outside the forum directory (as I presume your portal is) you need to change the relative links in a style to absolute links.
ie: By default, the buttons path is /images/steelblue/buttons. Because your page is not in /forums/, it is looking for /root/images/steelblue/buttons/, which does not exist.
Go into your Style Manager and add
http://www.domain.com/forums (or appropriate) to the paths in the style. Dont forget the gradients.