I'm assuming that you want the header image to extend to cover the white border on the sides of it? It IS possible to do that by editing the CSS and using an image that is the exact width of the new sized space, but tweaking the CSS on layout things like this can be very fussy and difficult unless you really know what you are doing.
One way to do this is to get rid of the padding in the following CSS declaration - change the 20 to zero:
background: none repeat scroll 0 0 #FFFFFF;
padding: 0 20px;
But as you can see if you try it, that moves the dark grey in closer -- and then there is no space between the text and the dark grey -- which does not look so great. So then you would have to change the margins or padding on every single element that is on the edge of a page. Not a good way to go because of the many places you would have to change things, and you'd likely still have problems on some pages.
The other way to go is to make the header section wider (both the image and the navigation bar) so it extends to the dark grey -- covering the white borders.
To change the nav bar width, you'd change the width in the below CSS to whatever width exactly covers up the white -- looks like it is 980.
background: none repeat scroll 0 0 #000000;
margin: 0 auto;
You'd also have to use a header image that is exactly that new width and change the width attribute in the image tag.
Those changes MIGHT do what you want, but there might be other problems on spacing or layout that you would have to deal with.
Also, if you are using different templates for different layouts, you might have to make these changes in several places -- depending on how the theme is coded -- if all the headers and overall layouts are the same, then doing it once should work.
But like I said, this kind of thing can be messy and tedious.
You'd really be better off looking for a theme that has a header and page layout that you like as is.
And for very, very sure, do not make any of these changes in the regular code -- definitely use a child theme so that you can easily "erase" your changes if you make a mess of it!