search through the wordpress directory, the link will be in your admin cp under the themes area (now design in 2.5). I'd recommend blue earth 10, I used that one and completely changed it. As for changing the header you can do that anyway, you just have to look for it in the header. I replaced ours with an image. If you want to see our modified theme, here's our website:
http://www.itbgames.comif you want to see blue earth, here it is:
http://themes.wordpress.net/columns/3-columns/4190/blue-earth-10/I will say this, if you want to modify a template, it's easier than making your own, and faster, but you won't know where everything is.
