Header background opacity help

Hello.

Greetings to the community, I’ve been struggling for a few days now trying to modify the opacity of the header background and I can’t do it, I just want to make the header back have an alpha of 50%, I’ve tried with some direct CSS codes but I’ve only been able to modify it when the header enters mobile mode, I don’t want that alpha to apply to the menu elements or logo, any recommendations?

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