Content width Clean Retina

I am using the Clean Retina theme, and I want to change the content width. I have looked at other threads, but the codes won’t work because it’s not the same theme.

My site is https://www.kimlovesthesmokies.com and I want the content width smaller, so the blue background will run along the sides as well, not just the header and footer.

Does anyone know a code or plugin that will do this?

Thank you in advance for your help!