There are no theme options to change the colour of the content area’s background.
The background of the content area is set in each skin’s stylesheet and is created with images. For skin 1 those images are in the ‘images’ folder, they are easy to spot because the filenames all begin with ‘pageBg’. For other skins you’ll find equivalent images in ‘images/skins/skin-#’ folders. To change the colour of the content area you’d need to create your own background images.