Cita: <Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="/Xceed.Wpf.AvalonDock.Themes.VS2010;Component/Theme.xaml"/>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
Intentan cambiar la ruta:
Cita: Source="caperta\Theme.xaml"
Espero te sirva!!