Modifying the default WordPress theme

I’m trying to modify the default WordPress theme, and in particular the header. I’m trying to get a handle on making the blue header block larger or smaller. I don’t really understand because it appears there is an image file, but that you can also change the color of it?! It’s the blue block at the top defined by CSS properties or is it an image?!

Can someone explain to me how it works, and what I need to do in order to shrink or grow the size of the blue header block?

Thanks!