In your style sheet, "style.css", the following style definitions exist. The first one sets the overall background color to white (#FFFFFF). The second one is for the center of your web page. Because it has not background definition, it inherits the white background from the "body" definition.
Code:
body {margin:0; padding:0; background:#FFFFFF ; height:100}
.site_center1{margin:0 auto; text-align:left; width:766px}
To make the overall background black (#000000), but leave the center of the web page background as white (#FFFFFF), make the changes below. This sets the overall background to black, but defines the center as white.