Thanks. The middle column, though, instead of remaining white, also changes to the background colour.
You will have to edit includes/templates/YOUR_TEMPLATE/css/stylesheet.css (YOUR_TEMPLATE refers to the name of your custom template) to separate out the middle column from the rest.
Try adding this to the stylesheet (replace the content in red with your desired color).
Code:.centerColumn{ background-color: #ffffff; }
Thank you - yes, that worked.