WPF-Demo/Waaagh/Themes/Generic.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:Waaagh">

    <ResourceDictionary.MergedDictionaries>
        <ResourceDictionary Source="Waaagh;component/Themes/CustomControl/WaaaghWindow.xaml"/>
    </ResourceDictionary.MergedDictionaries>

</ResourceDictionary>