How can I do that the background in the body class (body{}) will be over all the object in the page? (Example - if the body background is black and there's five objects in the page with white background, all the page will be background)
Why in the world would you want to cover your content with a background? That defeats the purpose of the BACKground and renders your content virtually useless.
If you really want to accomplish what that site does, have a look at that site's HTML source and the CSS. That should put you on track.
If you move the background to the foreground, well, it's not the background anymore but the foreground (duh). So, just use a foreground image of div or whatever you want to have on the foreground. Then use z-index to layer everything.