You’ve got to be careful here. Computer monitors don’t function like television screens, so 200px margins on your computer could make the center portion very wide for someone with a large monitor, or very narrow for someone with a small monitor (as on laptops).
That said, what you are asking for is really difficult, because Graphene’s header is contained within the main container that determines the width of the theme.
IIRC, someone around here did manage to pull this off a few years ago by using a custom background, but I don’t have a bookmark for that thread. If you search the forum, you may find it.
You could fake it with using a scrolling background.
Basically, you make the top of your background image look like your header, and then set it to “scroll”. Kind of a hack, I admit. To accomplish a true full-width header, you’d need to use a child theme.