I don't have the theme you are referencing in specific on my machine, and if you are using Drupal 7 it will be slightly different, but to use garland under Drupal 6 as an example, and to expound upon the above and comments below, if we open up its .info file, we will find a:
regions[header] = Header
This tells the theme engine that it has a region called header. So, if we look in its page.tpl.php file, we will find:
<div id="header-region" class="clear-block"><?php print $header; ?></div>
So, when the user puts blocks in the Header region via the Block Administration page, drupal puts all of the html etc into the $header variable for those blocks, which is then output here when it paints the page.
As you can see, $header is wrapped in a CSS id and class, so you can then look back at the .info file to find your CSS files:
stylesheets[all] = style.css
Opening up style.css reveals:
background: #d2e6f3 url(images/bg-navigation.png) repeat-x 50% 100%;
which gives the first of many cascading styles to apply to the theme's Header Region. Don't fret over the clear-block class, it's just a wizardly way of making sure nothing floats next to the header (and vice versa), and you aren't ready for that yet lol.
So if you do this with your theme, you should be able to discover how its header is styled.