Changing up the header

The header containing my main menu is on top of an image that is page wide. I’d like to just have a black band for the header with the menu there as it’s a bit difficult to read the menu on top of the background. When I go to the header builder, nothing I do seems to have any impact on the header itself. I’m using the Astra theme.

Any suggestions would be much appreciated.

The page I need help with: [log in to see the link]