I am using a remote desktop to my Windows PC which can use Visual Studio (Darn Ubuntu and no compatibilities with MonoDevelop/Visual Studio form designer) and I came across the problem of the header colors and content not filling out the content of whole site, but only up to the size of my explorer window.
My window in the remote desktop is 800x600 and is smallest I can use while not taking too much space of my original screen. It's not just the screen size either; it's the width of the browser window, so users of any size screen can encounter this issue.
This happens in all major browsers.
Example (as you can see the right top corner; it appears in every SE site):