Can't change second half of secondary menu color?

this is a preview of my website https://walkinginthedesert.com/

As you can see the color in the first part of the secondary menu(navigation bar) is black, as I had changed the color by modding the menu-secondary-bg.png image.

The problem is that the buttom part of the secondary menu does not become black as well. Is there something else I have to do?
In other words how can I change the whole part of the secondary menu and make it black, and not just the first half? I have been trying to do different things, but nothing seems to work?