Superposition of layers in new theme

Good afternoon,
I’m designing a new theme with Dreamweaver. I would like to know how I can do to set an order of superposition for “header”, “content” and “footer”. The idea is to set “content” below and “header” and “footer” above.

The reason for me to do that is that I want to create a shadow effect in the “header” area and this shadow would be casted on the “content” area, so I need “header to be first.

Thanks a lot in advance!