I have a WPF application I am developing, that allows the user to switch the current theme. I figured out how to switch themes... but it appears that the background of the application isn't affected by the theme:

(I blended three pictures together to conserve space)
How can I fix this? It clearly shows here that the background is supposed to change...
Here's the code I'm using:
MainWindow.xaml:
<Window x:Class="GDE.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525" MinWidth="525" MinHeight="350">
    <Grid>
        ....
    </Grid>
</Window>
App.xaml:
<Application x:Class="GDE.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary x:Name="ThemeDictionary">
                    <ResourceDictionary.MergedDictionaries>
                        <ResourceDictionary Source="/Themes/ExpressionDark.xaml"/>
                    </ResourceDictionary.MergedDictionaries>
                </ResourceDictionary>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>