User-customizable Forum Width?

[The Latinist]

I've got a forum community divided over whether our default theme should be fixed-width or full-browser width (or, rather, I've got small vocal minorities on each side of the issue).  So I'm looking to add a way for those users who prefer a fixed-width layout to have one, and I'm looking for the best way to go about doing this.

My first thought has been to add a checkbox to the "Look and Layout" tab of Settings to set a variable.  I could then rewrite the templates to echo either "width: 100%" or "width: 960px" in the style of the forum wrapper div depending on the value of the variable.  But I'd like to get some feedback on whether there's a simpler, better, or more elegant way to go about this.

[Illori]

why not have a fixed width theme they can select to use if they wish to?