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How to set FontFamily for an entire WPF application in a theme?

I have a project `Anonymised.Theme` which contains XAML resource dictionaries for a theme. This is used by about a dozen applications which simply include it by merging

    <ResourceDictionary Source="/Anonymised.Theme;component/DefaultTheme.xaml" />

into `<Application.Resources>` in their respective `App.xaml` files.

Now the designer has asked me to use a custom typeface rather than defaulting to Segoe UI. I have added the TTF file to `Anonymised.Theme` and embedded it appropriately with

      <ItemGroup>
        <Resource Include="Themes\Tajawal-Regular.ttf">
          <CopyToOutputDirectory>Never</CopyToOutputDirectory>
        </Resource>
      </ItemGroup>

The problem is **how to actually use it by default** *without* touching anything outside the theme.

### The non-solution which "works"

I can add

    <FontFamily x:Key="Tajawal">pack://application:,,,/Anonymised.Theme;component/Themes/#Tajawal</FontFamily>

to the theme XAML and then manually set

    FontFamily="{StaticResource Tajawal}"

on *every single window* in *every single application* which uses the theme, but that's almost completely defeating the point of defining a theme. In particular, it creates a massive maintenance headache, because then I need to remember to do that every single time I create a new dialog in the future.

### An almost-solution

Less of a maintenance headache, but still not really a true solution, is to add the `<FontFamily>` to the theme XAML as before and then make an edit per application to override the default style of `Window` by adding the following to the App codebehind:

        public App()
        {
            InitializeComponent();

            FrameworkElement.StyleProperty.OverrideMetadata(typeof(Window), new FrameworkPropertyMetadata
            {
                DefaultValue = new Style
                {
                    Setters =
                    {
                        new Setter(Window.FontFamilyProperty, FindResource("Tajawal")),
                    }
                }
            });
        }

### Things which don't work at all

Although it feels like it should be the same as the almost-solution, the following XAML:

      <Style TargetType="Window">
        <Setter Property="FontFamily" Value="{StaticResource Tajawal}" />
      </Style>

(in the theme or directly in `App.xaml`) doesn't appear to be used at all.