I got one more question regarding this topic:
and how does it coop with UI Resources module. Can you “just” install a Atlas UI Resource module from Mx7 in a Mx8 project?
Ok I recieved an email from Danny which indirectly answered some questions.
If I understood correctly this is what he said:
AtlasUI at some point will switch to the Mx8react folder structure (expected June 2019).
To start styling an upgraded Mx7 project it is advised to copy the styling from the Mx8react project by exporting the AtlasUI module and importing it in your upgraded project, this should ensure you have the right file structure in your themes project.
Thanks for looking into this Jason!