I want it first to be transparent, and when you scroll it should get black background like here:
https://preview.likeaprothemes.com/#nowadays_wp

There is no topic for this theme, but my client installed this theme so I have to use it.
Can someone help me with that?

When I use your link, I see a completely transparent header menu and then on scroll a dark block: background: rgba(11,18,22,1) !important;