Thanks for answering.
I tried what you suggested. Only one problem:
I'm not sure whether I added the style correctly to the player. Here's what I did:
var bufferingPlugin = player.Plugins.OfType<BufferingPlugin>().First();
bufferingPlugin.BufferingViewStyle = App.Current.Resources["MyBufferingViewState"] as Style;
Excecuting these lines of code results in an TargetInvocationException.
I did a debug session and found out, that "MyBufferingViewState" is correctly set as a style for the BufferingViewStyle. I just copied the style from the generic.xaml without editing it.
(Thanks for showing me the generic.xaml, it will be a perfect help for my future design-related problems)
Any ideas on that ?