well....it is controlled in your themes master.css file. First off, there are some instructions at the top of that file to not adjust the css in there....as it will break when you upgrade. So you have to decide if you want to take the simple route, adjust the css in master.css and have to redo it if you upgrade the theme. Or you can follow the theme authors advice about child themes. That's all up to you.
As for your original question.....there are 3 sections in your header you can adjust....
I believe they are as follows...
#header {
width: 100%;
border-bottom: 1px solid #EDE7E0;
background: #443B31 url(../images/nav-background.gif) repeat-x bottom left;
}
this controls the large section, the background behind your sites title and description. The hex is the brown I believe, there is also a gif image overlayed with transparency that you may have to recreate. (it looks like that creates the slight gradient in you navigation area.
the next bit of css is this:
#follow {
background: #2F2922;
padding: 5px 0;
}
That one is the very top brown bar, where your follow icons are located. You can simply tweak that hex value
Finally you have the nav section. Like I said, I believe the gif image in your #header area controls the gradient, but the hex values throughout the nav section control the colouring of the tabs on hover, etc
something you may want to consider, is using firefox browser with the firebug addon, I see a lot of folks on here recommend it.... it really aids you in tweaking css...
http://getfirebug.com/