Forum performance -- forum.css is included on every page

Looking through forum.css, it seems like it's probably not necessary to include it on every page view. Doing so:

* slows down page generation with extra code getting called during the menu hook on every page.
* uses up needless bandwidth
* makes it take longer in some browsers for anything to show up on the page at all -- some browsers appear like they might wait until all the CSS is loaded before starting to display anything.