Its a new forum theme and believe me we who tried it said there would be complaints with the fixed width.
So much so that there was another theme crafted for variable width, AV6 evang, now I don't know if this theme is available in the user Profile, Look and Layout Preferences, Check the current theme, it will probably say Forum Default. Click the (change) link and see if the AV6 evang theme is available, that has a variable width.