You'd need to modify the theme:
What's it used for? Other than to make the page narrower?
Thats just it, it toggles between fluid and fixed width layout.
@baris Thought so. Cheers.
@baris Is there a way to keep the max width boxed instead of the width stretching out to the screen edges? Sometimes mine defaults to boxed and sometimes to wide (is boxed==fixed and wide==fluid?). I'd like it to stay boxed when the window is maximized and be fluid when the window is made narrow.