Transparent Header Bar

Hi there, On my site (http://www.zig-zag-um-die-welt.de) I do have a static solid black Header Bar on the Homepage as well as on any Posts. I would like to change it to the "transparent/ solid" behavior which you can witness on this site: https://astroforumspace.com/ I will try to describe what I have in mind: When completely scrolled up, the Header bar Background should be transparent. When not scrolled completely up, the Header bar Background should be black. Is this possible and how? :-) Thank you and best Regards, Pferdi