Sticky Header

Hi Forum

I ate most of my nails already, but i can’t get the header of this website to stay sticky and not change position when scrolling down.
Facts: -in chrome, developer mode, there are 3 lines in stylesheet if disabled, the header stays sticky, and moves nowhere.
/*
.site-header.float-header {
background-color: rgba(0,0,0,0.9);
padding: 5px;
margin-top: 0;
}
*/
-When The lines are blocked in Appearance/Editor like so, it doesn’t do anything. the header still slides.

Is there a delay between site update and refresh when working in editor?
There’s a transition happening when you start scrolling that I can’t deactivate.
The theme comes with some customizer options for the menu as well, I start believing that the line of code that the customizer uses is responsible for this unreasonable behaviour…

Forum, Please help
best
Peter

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