Is there a library for some kind of „native looking ui“? I don't really know how to describe this theme 😅 It's something you often see in some big data management or accounting programs.

https://github.com/kirill-grouchnikov/aurora maybe?

not exactly what I've been searching for but it's worth a try

Native as in full fidelity to the visuals of the underlying operating system (Windows, macOS, Linux flavors, etc) - that is going to be highly unlikely. Aurora (I’m the author) doesn’t aim to do any of that, but rather aims to provide a set of desktop grade components with built-in styles, as well as flexibility to provide your own styles.