Hey Tom,
I went through he same problem today

I only got the top and bottom of the header image correct for adjustments. Look for the following in styles.css:
.header_top {
background: url(images/header_top.png) left top no-repeat;
height: 24px;
}
.header { height:200px;
background: url(images/header.jpg) 23px top no-repeat #D6DBEB;
}
.header_bot {
background: url(images/header_bot.png) left bottom no-repeat;
height: 24px;
I changed the height to 12 px and the borders became thinner (the way I wanted it), HOWEVER, only the header_top and header_bot I could change. No way to find the url(images/header.jpg), so I gave up and modified the entire (954x200) images I use for the header.
In
Picasa (free download, and I use it for many image editing) I added the 'Museum Matte' look with a relatively wide border to the images I use as header, and deleted all that had to do with the header in my html pages.
In 'Museum Matte' you can select any color you want as well as the width of the border..!!!
In the html pages, I deleted:
<div class="header_top"></div>
<div class="header">
(changed this to <div align="center">)<div class="header_bot"></div>
Looks more clean and 'slick' with the much smaller borders...

Cheers,
Arthur