The general background of the webshop pages consists of a color, a background image (as a one-time image or as a pattern), or both (background color and image). In the latter case it's only usefull when the background image / pattern has transparent parts. More on that later.
Because the header, the middle part (the content) and the footer have their own backgroundcolor settings, the general background setting only will be visible in the rest of the page, so mainly around the content.
Setting the background
See 01a. GENERAL: Look & Feel - Background
Background with images or pattern
Background with color and image
In case you want to use a color as well as a background image, then that is possible but only if the image is partly transparent. Through these transparent parts the color is visible. You can use GIF or, even better, a PNG as file type. JPG has no transparancy.
Only PNG or GIF can contain transparency. Make sure your background image has Gif or PNG as file type in case you want to see the background color throught the transparent parts of the background image.