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"

There are various textareas where you can put in CSS styles, at Theme Options -> Post/Page styling

Centering the page menu bar is described at http://forum.bytesforall.com/showthread.php?t=122 post #19

What do you mean with "...text in a new post..."? The text size for posts can be changed at Theme Options -> Post/Page Styling -> BODY Box:

font-size: X.Xem;