Centering an oversized header

My site is http://www.lettersfromtheloft.com

The theme I’m using has a width of 960 px. I want a header of 1150 px. I’m able to make that change without any problems. (The current header size is 960 so it doesn’t look funny while I’m figuring out the next part.)

What I can’t figure out is how to get the header centered. When I set the header size to 1150 and upload a header of that size, it aligns with the left of the theme. Then the overflow is all on the right. I want the overflow equal on both sides.

Is it a matter of changing the margins? Will margin: 0px 0px -190 px 0 px work? If so, where do I put this?

I’ve been working with the stylesheet page and the custom_header page. I don’t see anything to change in the custom_header (except the width in one spot).

But here’s the current stylesheet code. Any help would be appreciated! 🙂

[CSS code moderated – the link to your site is enough to access the stylesheet]