I use GreaseMonkey (other plugins of the sort can do too) to override the width settings of #JelloExpander (set it to auto) and .threadPageContentContainer (set to any width that fits your screen. I don't set it to 100% because it makes the thread content
drop below the sidebar. Maybe set it to something around 80% can do. I'd appreciate if the site admins can add multiple @media override with min-width for us).
The VS style header can be expanded by setting ".header-top-bar, .ux-footer" width to auto, but there are other variants such as the Technet and SQL Server one. (I have no idea about the total number of themes on this site)
Here's how to add custom CSS override with it.