Hello,

is there a way to center an image in the event view when you don't have a fullsize image that fills all the space?
E.g. here the picture is aligned with the left side fswue.de/veranstaltungen/pen-paper-sunday-milan . I'd prefer it to be centered.

This is supposed to be, but due to the extension JCHOptimize loading a display block on all img tags, this is why it is not centered.
Add this custom css (could be done in iCagenda option for that, in the options of component, General Settings tab (at bottom).
This CSS:
About;

Another thing: Can I change the color of the navigation buttons? I've got a dark theme on the webside. Therefore the numbers on the buttons are white on white. fswue.de/aktuelles

This is not iCagenda list of events, but Joomla core articles
Look inside the css of your site template.

Best regards,
Cyril