How to make a central background over the page in HTML/CSS?

I want to add the content of a page to a part of the background. That is, I want to keep 2 unutilized columns of area on both sides of the main content. Something like this,where you can see 2 unused bluish areas on either sides of the main content. There also is a shadow effect seen. So the main white background area where content is typed must be an image. How do I achieve these things?I need to know how I can actually put another sheet over the whole page seen in the screen.(I'm referring to the 2 unused columns in the link ).
 
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