when you scroll down the page and simultainly the menu changes? Which theme can handle this?
Do you mean like Screens?
Yap! This is right direction. Maybe like this site: http://www.augenaerzte-am-theaterwall.de/
because it’s more fluent, or is it customizable?
Yup, you do not need to have the snap scroll turned on. in fact we tell people to turn it off on mobile.
Cool! Thx! 
Magicgellan 2 does this as well (see here: http://demo.bigwhiteduck.com/magicgellan2/examples/horizontal/) but Screens is an excellent product (and may be more ready to go ‘out of the box’).
Hi again!
At least I am pretty unsure now, because Screens with another theme or MagicGellan and FOUNDATION looks very good.
I have the impression, with foundation I am more flexible, wright? Any good tips for that?
The theme for Foundation is free so you can use Screens and Foundation. If you want the whole package then yes Foundation and MagicGellan is a good way to go and will give you more options