Change Theme of .NET 4.5 WPF Ribbon

I am trying to change the theme of the new WPF Ribbon Control from .NET Framework 4.5 and I am stuck.

I only managed to change some brushes (Background, Foreground, Border ...) but I seem to be unable to change the bright overlay and shadows.

I am happy to use a resource dictionary but I don’t know which properties I need to set.

I hope you can point me in the right direction, thank you much for your help!