Hi Rolf,
It’s difficult to say since your site is in maintenace mode and so I can’t see what’s happening – plus, I don’t know what you actually added to your custom stylesheet.
The first thing is to ensure you are targetting the element correctly. This won’t work, for example:
tribe-events-single-event-title summary entry-title { color: #fff }
It would need to be:
.tribe-events-single-event-title.summary.entry-title { color: #fff }
If that is pretty much what you have and it still isn’t working my guess would be that another more specific rule is taking precedence, so try tightening it up to something like this:
#tribe-events-content h2.tribe-events-single-event-title.summary.entry-title { color: #fff }
If even that fails, I’d really need to see a live example to help further 🙂