Hi Jorg
I’ve not been using my multisite network for very long, ie I’m no multisite expert! However, in order to activate either a theme or a plugin indivually for a site you need to leave it Not Network Activated, then head to Network Admin>Sites>All Sites>YourSIte>Edit and for themes click on the Themes tab and activate the theme you want there. For plugins go to Network Admin>Sites>All Sites>YourSIte>Edit>Settings and scroll all the way to bottom. You will find a list of your non network activated plugins there and you can again individually allow a plugin to be used on a site.
Once you’ve checked the tick boxes for the theme or plugin for a particular site as above, then you’ll need to go to that site dashboard and activate the theme or plugin there.
Hope that helps!
Ali-B