“I think I found it: the lightcase and the UIkit3 atoms have to be turn on. That’s it, right?”
For modal module you just need Uikit3 atom :), its correct.

“Would be great to have a reminder in the particles, if an atom is needed to be loaded. Though I know, this is lots of work.”
Most particles are using Uikit 3 js/css so you need to enable the uikit 3 for gantry 5 atom first. We already mentioned about this in our documentation https://joomlead.com/gantry-5-particles/particles-documentation/#first-step
For next time, please help us submit technical questions via our ticket system.
Thank you