Changing color scheme twentyeleven

Hi,

I want to modify the color scheme of twentyeleven. So I created a child-theme as described in
https://www.ads-software.com/support/topic/editing-twenty-eleven-color-scheme?replies=7

I activated the child-theme and started to customize using the wordpress dashboard.

Issue no 1. What happened to my header image and why can’t I change it to a custom image that i can upload? Why no upload at all?

Issue no 2. I guess I’l need tot add some stuff to the child-theme’s style.css. Will this override certain stylings of twentyeleven’s style.css?

To be specific, I want to change the color of the menu background from pitch black to dark blue and do some other editing.

See https://kaa-s.nl