You have to add a reference to "PresentationFramework.Aero" in your project, and then add this to your App.Xaml file:
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="/PresentationFramework.Aero;component/themes/aero.normalcolor.xaml"/>
</ResourceDictionary.MergedDictionaries>
<ResourceDictionary>
</Application.Resources>
You could have easily found this on google.