Background- do I think correctly ?

I am new to CSS, and I am trying to figure out how it works. I am developing my web page www.projektowaniebim.pl and next thing I would like to do is to play with my background.
And here is my thkinking process and understanding of it, tell me if I am right:

1. In content.css in body i entered parameter background: #something - (I don't know actually if the value I enter have any influence on the look of the page, or it just have to be defined to be used ??-this is a question).

2. I want to have a color only on the sides not on the area with text (posts), so I should do something with my grid layout and create some kind of columns (??) and than apply the color to them ? I tried to add background: #something to layout.css and it applied it to the whole width, so it seems as if I had one big column (??)

Can you tell me if I'm thinking clear ? Or I totally don't understand the concept, if so than can you give me some kind of push in the right direction ?