Header Background Opacity

I’m trying to create a custom css code to change the opacity of our header menu background color (so you can see the hero image behind the menu)… Just like this website: https://www.digitalvortechs.com/
Does anyone have a code for this?

We’d like it to be When you scroll down the page, the sticky menu opacity would go back to 100%. (Again, just like this website: https://www.digitalvortechs.com/)

Our website is: https://newp3.p3post.com

We purchased the OnePress Plus plugin.

Please help!