How to add a solid border around the canvas of Hudson template?

I am using the Hudson template and would like to add a solid border just around the canvas area, somewhere in the 4-6 px range. This site is an example of what I am trying to achieve. I tried inspecting the code but nothing I placed produced any results. I do have the "page borders" turned off in my template settings, and would like to keep them that way. Is it possible to add just a solid canvas border with CSS?

Thanks!