How can I make the javascript work for the default forum page and have individual headers for other forums?
Okay, I just noticed this.
You may have to settle for including specific banners in the "Forum Intro" section of each forum at Control Panel » Forum Settings » [FORUM NAME/Details] » Forum Introduction Body. That would, however, only include the banner on the forum's postlist page and not every page...
Maybe someone else has a better idea.