I've made it the same as Body, and that did it. I also removed the radius from the top and right borders of Navigation Tabs Container and added left and right borders to the #logo class in my EXTRA.css template to take care of the little "overflowing "edges. I think that looks better.
The only problem I see is that it seems to make the forum not responsive. Some areas like the member card are better, but not by much -- text still gets hidden. Is it possible to re-add responsiveness while keeping the edits I made?
I noticed a small side effect of using this specific width. When I over the items on the edge on the moderator bar (Reports and Admin Control Panel, respectively), the hover background kind of overflows into the HTML background.
Is there any way to fix this? Here is a screenshot (it's the green):