Can’t Change The Header Menu Color

Hi every one,

I’m having problems with the header on this website. I would like to add a new cover image to the header, but the menu-color will not change. I’ve tried playing with the theme’s customization, and tried with CSS, but I can’t get the menu-color to become transparent. Does anyone know how I can fix this? ??

Sincerely.
Simon

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