Cosimo nav bar and post opacity

Hi! I’m using the cosimo theme for wordpress and I am trying to make the nav bar at the top transparent so you can see the picture below it. I would also like the post to be transparent. I want the header picture to take precedent and be able to see all of it. Any thoughts?

When I do this
.site-header {opacity:.5;}
in style.css it makes the actual picture 50% transparent.

Thanks!