WPF Themes not applying to background - wpf

Question

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>

Answer 1

The themes define a background but you need to set it up yourself since it is not being referenced anywhere within the theme:
Background="{DynamicResource WindowBackgroundBrush}"
Why? I'd assume because styles are not automatically applied to derived classes, so if a style with theTargetType Window is set up that would also need to be set explicity since normally you use a subclass of Window, e.g. MainWindow.

Answer 2

My guess is that that the background we see in your screenshot does not belong to one of the themes, and this is the problem, you override theming when setting stuff manually.