WPF-Demo/Waaagh/App.xaml

<Application x:Class="Waaagh.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:Waaagh">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="/WaaaghTheme.Resources;component/DarkTheme.xaml"/>
            </ResourceDictionary.MergedDictionaries>
            <SolidColorBrush x:Key="TestBrush" Color="#669999"/>
        </ResourceDictionary>
    </Application.Resources>
</Application>