Change the color of the Body

I have made my site i bit smaller and set the with to 890px. Now it has to areas on each side that i want to change. Is it possible to have one color of the background of the "post" and another color on the sides.

I've tried to change the color in the "Body" but this changes both the sides and the "Post"-area

Question Nr.2
Can I make the text in a new "post" a bit bigger. I find everything else but the textsizing

Question Nr.3
Is it possible to center the "boxes" in my horisontal "pages-bar"