I'm on a MacBook Pro 17". I can confirm that my pages look pretty much the same as yours (as does my 'Screenshot 2' above), right down to the fact that the content is slightly to the left of centre.
The question I need to ask, and I'm happy to be embarrassed by the answer, is what are you doing for a forum search function? I can see a magnifying glass icon next to the logout button near the top right of screen for a brief moment when the pages are loading (see my screenshot 1 above), but this icon disappears when the page is loaded and useable (the icon, and I'm guessing it's the forum search icon, is also missing from your screenshot).
While you're at it, could you fix the CSS (or whatever) so that the entry text box can be delineated more clearly (see attached image). There's no boundary on the left, right or bottom. I see this issue on both Firefox and Safari and it's sometimes hard to see where to click to start typing a response.