Hi David, thank you for the quick reply.
The answer is: yes and no.
First, I had just assigned one font to body in the typography setting.
Everything was shown in this font, except the header (=primary menu).
Second, I tried to fix this by additionally assigning the same font to “Primary Menu Items”. It didn´t change anything.
I meanwhile have rolled everything back to a backup version without the library template, that made the changes.
Before I had imported a template from the generatepress library it was possible to change the font in the header.
Is there a way to find out, how a library template is changing the “fundamental” settings of my project?