Small CSS question

Hello all,

Since a few weeks I’m working on my new theme. But I don’t have as much knowledge of css than design.

It’s about Another Blog, my blog.

As you see, I have used this picture as a body background.
Now I want the upper part of that background, let’s say the header (the grey/black part), to be horizotal repeated. But it just doesn’t work..

You see, my main problem is that I have this header, but the body part of my design overlays it for a small part. How can I do this best in CSS?

This is how it is meant to be: click.

I hope I was clear enough.
thanks in advance,
Robin