Border Around Entire Content Area

I've done a lot of searches and maybe this is obvious (or at least I'm thinking it should be obvious and easy to accomplish), but how do you create a border around the entire content (header, content, footer, etc.)?

I would like to make my max-width about 1200px, then put a border around that container. Then I would like to add a background image to the area behind that 1200px wide container area.

For some reason, I'm having a hard time doing this within the customization area. Is something like that not supported natively? Will I have to add some custom ccs?