Set background colour of area outside of content when using boxed layout

I feel silly asking this question but I can’t seem to find it anywhere:

If I set the theme to use “Boxed Layout” – where can I set the background colour (that is, the areas to the left and right of the various content areas?) Is there a way to set it to an image?

Please note: I do not mean the “default background color” as set under the “Main Content” tab but the actual background outside of those areas.

Thanks in advance.