[kh99]

I really don't know a lot about styles, but I don't think you can use an 'if' template tag in additional.css, because I think the styles are pre-built and not rendered for each page like other templates. But what I think you can do is create a child style for each of your forums and use the "Style Options" in the forum manager to assign a style to each. Then you will have a separate additional.css for each forum.