Hi,

1.Is it possible to build a own theme that I can use in the extension?

Yes, but you need to know HTML and CSS, Social Network have own theme manager to override and create template file.

2. Can I modify or configurate the extension so, that only messaging and the event-wall will be available for users? So that users are only allowed to create an event or join an event?

No sorry, you can disable some parts like photos,blog,topics and videos and groups but mainly you will have always social profile page and wall.

So that users are only allowed to create an event or join an event?

Social Network plugin uses a own role and plugin system, you can set following permissio about events
- Event plugin: global access (based on roles)
- Role system: create/edit event permission
See screenshot
Note: you can't set permission to RSVP

We want our customer well informed about our products, so if you want tomorrow we can provide you a private demo