Where can I set the color of the menu text background?

Where can I set the color of the menu text background? At the moment it’s transparent. It only goes black if you scroll down.

And how to add a Menü Point and categorize the pages under that menu point?
Like: waterfall
waterfall venezuela
waterfall spain
waterfall USA

And that the menü shows only waterfall. And when you click on waterfall that all the pages are listed. Like if you would use Category with blog posts.
But how to do this with pages?

Hello there,

To change the default header menu bar background color before scrolling state, add this CSS code to Appearance > Customize > Additional CSS from dashboard.

    .site-header:not(.float-header) {
      background-color: #fff000;
    } 

To create sub menus, consult to this codex.

Regards,
Kharis