how to add border/background to posts?

Could someone tell me how to achieve this? I want it to make the posts have an background and a border. Kinda like we see on this forum. The blue background is the main background of the site, and the forum has a lighter blue or gray background around it with a darker blue border. I want to do this for my blog posts. Can someone tell me what code I need to edit and what I need to enter? I can figure out the color changes.