Background Content

Hi there, I would like to do an image of transparent background over the black background and only in the content area. But I tried and tried and I do not found where I could to that?
I made an image in the width of the menu-ribbon and left an right ist empty space so that that only in the middle behind Content and sidebar it is white (wish-color).
I bought this theme. Please help. Thank you very much...
engeldeluxe@googlemail.com