Is it possible to adapt the menu in the Deliver theme to behave like the menu in the Gravi-k or Animatter theme? (fixed on top)?
Anything is possible. You just need to get your hands dirty with the HTML/CSS. There’s no simple, single-click way, but you could largely copy the code from one to the other, depending on what framework they’re using.
Thanks Perlkönig, css is not my forte; I was hoping for a configuration thingy in the modular.md/corresponding twig-file 
If the theme doesn’t document such a setting, then it probably isn’t there. You could always reach out to the theme designer and see if they’re up for the challenge.
I might just do that.
I had some bad luck in that respect with the Gravi-k theme (I’m even willing to sponsor development of it to get it up-to-date) but the developer is hard to reach (or has abandoned it all together)
The author does seem to be responding to GitHub-issues. If they are slow, it might help to post a new comment to the issue tagging the author to get their attention, after you have tried new solutions.
This is at least more likely to be seen by authors of themes and plugins than through the forums or Gitter.