This style has a wrap_boder.div. Without it in any case. But with the wrap border you have to specify the maximum width. I have to see if this works with a percentage. (The brown border is a div behind the whole style).
Edit:
With outwitting, it worked. But I still have to look at the responsive to see if it's right everywhere.
In addition, the header image would also have to cover the entire width, otherwise there would be no navbar and no breadcrumbs, because these are created by the header image. You would still have to specify a maximum width in the width of the header image. Unless you find it good without a navbar.
Or insert an additional div so that a background colour can be given.
Responsive it is, if you are interested I would have to know if with additional background colour or not. Because a few adjustments have to be made for the style in full screen.
Off topic: question was good 
, I only saw now that the header image is not centred - 2px more space at the bottom
Edit:
Of course it looks better with the background colour like in the forums.
Keep in mind, however, that the displays are getting bigger and bigger. Full-width in my display is quite cumbersome. Therefore, you should still specify a maximum width.
(no max-width for the wrap in this file)