No, the width wouldn't be global for all styles, even if I make it ACP configurable. There are styles with a 700px (or even less) centered table. You can't have the same max width for those as for exemple the subSilver style which fill up the screen width, whatever it is. Login to the test account on my demo board and change the style to Skaidon, you'll see what I mean. The max width for Skaidon is set to 150px:
max_width for each style installed on this board:
Skaidon (150) this design is buggy anyway
all the other styles (400)
I would add a field in the Theme Settings page. You would change the max width for each style just like you change the CSS Stylesheet and colors on this page.
The max width for each style would be saved in the database and retreived along with the other board style settings whenever a page is loaded.
You're lucky, I'm bored this week-end. Maybe I'll code it. But it's no picnic... I've never coded a MOD that alters the database. I guess there are a whole bunch of MOD standards to follow for that as well. I'll have a look at how the others do it and get back to you.