Read original post by András Velvárt at Dotneteers
The built-in resources in Windows Phone 7 help a lot with automatically reflecting the selected theme of the phone. (See my earlier blog post: Being theme aware in Windows Phone 7 Silverlight apps). However, sometimes you want to be theme aware, but need more than the predefined resources. For example, you may want to have a generally blue background color, but want it to be a dark blue for the dark theme, and a light blue for the light theme. I created a little Behavior that allows you to do just this.