Editing Header Images in a Theme

I’m using the theme “Childishly Simple” because I like the 3 column layout available. I know CSS pretty well, but not if I can’t find the applicable CSS! I need to center the Header image in the theme and for the life of me, I can NOT find the CSS code to allow me to center the Header image! Typically centering an image is quite easy…if you know where the applicable .CSS code is! Can someone PLEASE tell me where to go to get to the actual image url so that I can get it centered on the page?