By the way, why is the forum fixed to a constant width?
This is how the theme works. Other themes, such as the default one, have a variable width.
However the current theme can easily be modified to have a variable width too. So later I can provide two versions of the theme: fixed or variable width, and each user can choose its preferred option.
Personnally, I think that very large lines tend to make reading harder, so it's not an issue for me.