This can be achieved using custom CSS. The header, sidebars, and post are contained within a “.wrap” selector. Setting a background color property on that selector via custom CSS will fill those sections. For example, if you’d like to have a white background, use:

.wrap {background:#fff;}

If you’d like to adjust the left, middle and right columns individually, you may use the “#sidebar”, “#primary” and “#secondary” selectors respectively.