Help with separating page elements

I have a question that seems very basic to me but after an hour of googling I just can’t seem to find a solution.

Basically, I want to separate the page elements on my blog (header, posts, widgets, footer, etc…) and have space between them where users would be able to see glimpses of a background image. Essentially something like this:

http://www.rockpapershotgun.com/

Right now I’m using the Thesis 1.7 theme, and I’m fairly sure that it’s possible to do it using custom css or something, I just have no idea how.

Please help!!