Transparent header

Hi,
I know there are some topics about a transparent header, but none of theme helped me so far.
On my localhost I’m working on a website, wide responsive layout.
I added a background image and the goal is to give the header menu a transparent background color, so you wil see some of the background image.
I tried all kind of css changes and I’m also familiar with the ‘transparancy’ option per page. This could potentially work, however I have more than 300 pages and I need a general solution for this in the code or css.

Thanks in advance!