Javascript: Move entire site down 15 pixels and insert banner

I am creating a client login for them to view their site during development. But what I want is to inject a small banner at the top of each site page with some information.

What I want is the site to be pushed down about 15 pixels and the banner be there. But I can’t find a guide on how to do it.

My other option (the considerably easier one) is to just to create a set of frames and load the banner into the top and the site into the bottom.

But I would prefer use Javascript since I never done it before. If anyone knows of a tutorial that would describe how to get Javascript to do something like that would be wonderful.