SMTheme prevents MingleForum CSS from working right

Hello,

I use the Hockeytheme and want to use a plugin called Mingle Forum.

It's a plugin which allows me to add a complete forum structure to wp pages or posts.

Problem: Somehow the CSS from hockeytheme prevents the css from mingle forum to work properly. Changing the forum skins doesn't help.

When I switch the theme off, the skin is showing properly.

How do I fix this?