Add border around the page and posts

Hi, Firstable, Thank you for the Catch Box Theme, it is very nice ! I would like a black border around my site and around the posts. I have try to draw that i want. I have do that with paint, it's just for show the idea. Here is my site : http://guitare-tablatures.fr/ Thank You very much ! P.S : I have already a child Theme P.P.S : Sorry, I don't speak English very good...