Graphene is not a fluid-width theme. This is possible, however it depends on two things: 1) the kind of header image you are using (is it repeatable along the x-axis?), and 2) how much do you know about CSS, or how much are you willing to learn?
This was actually solved some months ago in this forum, but it was not easy to accomplish. If you really want to do this, take a look back around a year ago, or use the advanced search feature.
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