Header: Font is fixed / Background Colour is not

Hello.

I made a fixed-navigation header with this code:

-----
.site-header {
position: fixed;
z-index: 10000;
width: 100%
}
.Site-inner { padding-top: 100px; }
-----
(The fat text "site-header" is the header class name defined in your template and should work for everybody.)

But if you checkout my website for instance https://www.kleindinst.at/dataprotection or the picture attached you can see that it's only the font that is fixed.
I would like to have a black background full width behind the header text that overlaps the rest of the website-content when I am scrolling.

It would be great if someone could help out.
My template is "Flores".

Best,
Anna