With xF, I appear to only be able to achieve this via the description that appears in the hover-over tooltip which doesn't emphasise it enough when a user is scanning/scrolling down the forums index page. I'm only new to xF, but I did have a look at the styles CSS and could only see how to have the same for all forums and not pick out individual forums.
Thus, I would like to suggest that within the Admin CP that the node tree for a forum include a input underneath the forum title that allows entry of html or css(preferably, more flexible?), so that each forum title on the forum index page can have an image of style applied.
Example showing where used a simple html img tag within the forum title to highlight it. ...