I see what you mean now.
I've actually tried to tweak on the unsleepable theme to suit what you want.
Unfortunately, you can't place an image on the background with just a few simple tweak, it will affect the header, just like what you said, the black thick border wont run through the entire top. However, you can still change the background colour by replacing #ccc to your desire colour.
I also tried to replace the order of <div>header and <div>page in header.php, and change the width of header to 100%, this is what I get -> http://niceric.com/simplicity/ , but by doing so, you will be able to add background image with the top black thick border still run through the entire page.
Changing CSS to your own taste can be frustrating, but also very rewarding.