Hi,
This is my website http://www.impressagirl.info
using Atahualpa 3.7.7 and WP 3.3.1

I'm trying to change the background-color of the post as you can see the orange color, but I don't know how to change that white space around the post so the entire column color be the same color.

Look at the style center area section and use a background-color

I did, in the style center column, but look what happens. http://ScrnSht.com/uqdvuz
You can see the black below white rounded square.
I just want that white rounded square to change color.

I think I got it. It's the style post & pages>post container. http://ScrnSht.com/ohranc
Correct me if I'm wrong

there are multiple parts, the middle is there (on top of the container which is on top of the page) with the post container on top of it with the post text on it. All of them can be colored

think of the russian nesting dolls

This has nothing to do with your question but you might want to to fix the position of the background image. That way it won't scroll. I just like the way it looks a bit better.