Justin Tadlock 2 years ago The short answer to that question is that you can’t do that with WordPress’ custom background feature. The long answer is that it is perhaps possible if the theme design had a set width. However, the theme design is responsive so that it works on desktop and mobile devices, so the width is flexible instead. Without a known width for the container that wraps both the content and sidebar area, I can’t think of a way to do it. Even then, you’d have to add a custom background to that container and not use the WordPress custom back feature. Your best option is to add a solid background color on the sidebar itself or the individual widgets within the sidebar.