Navigation bar should not be transparent

Hi there, I tried a lot but I can't find what's the Problem. I'm using Nepalbuzz Pro and want a white bar for my navigation. On the top it is white. But if I am scrolling down it's going to get transparent, whats pretty stupid, because you cant read the text then. My website is: https://www.muenster-ingolstadt.de/ Where can I change this setting or what do I have to do? Thanks, Best Markus