An easier method would be to remove the color from your columns and page color background and create an image that will tile underneath your site as a page background.
The very last feature in the Color Editor under Site Layout is the page background image. Click the browse button and select the graphic you made and upload the changes.
If you just want to have a color in the left column and then white or some other color in the center and right columns, make sure your graphic has the color you want in the left portion of your graphic stretch out to 180 pixels, then to the right of that place the color you have chosen for the center and right columns. The reason you need the left column 180 pixels wide is that that the default size for the left column in the

Commerce shopping cart.
Good luck!