I would like to know how to have a different main content background for different pages.
So I would like my home page main content background to be black and my contact us main content background grey.
The easiest way to do that would be to use either put all of your content on those pages in Color Sections via the Advanced Layout Editor. Next would be to use a plugin like http://wordpress.org/plugins/wp-backgrounds-lite/
Otherwise you would need to target each page by its ID in a css snippet and change the background that way. If you can provide a link to your site and page we can show an example css snippet.