I’m trying to create a ResourceDictionary containing icons that use Brushes defined in other dictionaries in order to dynamically load light and dark themes.

Icons.xaml

<ResourceDictionary xmlns="schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="schemas.microsoft.com/winfx/2006/xaml">   
    <DrawingImage x:Key="IconImage">
        <DrawingImage.Drawing>
            <DrawingGroup>
                <GeometryDrawing Brush="{DynamicResource SquareBrush}" Geometry="M0,0 L100,0 100,100 0,100 Z"/>
                <GeometryDrawing Brush="{DynamicResource CircleBrush}" Geometry="M25,25 A25,25 0 1 0 75,25 A25,25 0 1 0 25,25"/>
            </DrawingGroup>
        </DrawingImage.Drawing>
    </DrawingImage>    
</ResourceDictionary>

Light.xaml

<ThemeDictionary xmlns="schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="schemas.microsoft.com/winfx/2006/xaml">
    <SolidColorBrush x:Key="BackgroundBrush" Color="White" />
    <SolidColorBrush x:Key="SquareBrush" Color="Green" />
    <SolidColorBrush x:Key="CircleBrush" Color="Red" />
</ThemeDictionary>

Dark.xaml

<ThemeDictionary xmlns="schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="schemas.microsoft.com/winfx/2006/xaml">
    <SolidColorBrush x:Key="BackgroundBrush" Color="#1e1e1e" />
    <SolidColorBrush x:Key="SquareBrush" Color="Yellow" />
    <SolidColorBrush x:Key="CircleBrush" Color="Blue" />
</ThemeDictionary>

App.xaml

<Application x:Class="Test.App"
             xmlns="schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:Test"
             xmlns:icon="clr-namespace:Test.Icons;assembly=Test.Icons"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="pack://application:,,,/Test.Icons;component/Icons.xaml" />
                <ResourceDictionary Source="pack://application:,,,/Test.Icons;component/Light.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

This is how is used:

<Image Source="{DynamicResource IconImage}" Width="16" Margin="2"/>

This is how I select the theme:

private void LoadResources(bool dark)
{
    const string lightName = "Light.xaml";
    const string darkName = "Dark.xaml";

    string previous = dark ? lightName : darkName;
    string current = dark ? darkName : lightName;

    List<ResourceDictionary> dicts = Application.Current.Resources.MergedDictionaries.Where(ee => ee?.Source?.OriginalString?.Contains(previous) == true).ToList();
    foreach (ResourceDictionary? dict in dicts)
    {
        Uri source = dict.Source;
        Uri curUri = new(source.OriginalString.Replace(previous, current), source.IsAbsoluteUri ? UriKind.Absolute : UriKind.Relative);
        dict.Source = curUri;
    }
}

When I change theme, nothing happens to the IconImage displayed.
Other resources, such as the BackgroundBrush, load correctly.

What I doing wrong?

PS: If I remove Icons.xaml and paste the content to the end of Light/Dark.xaml, it work fine, but I thing that in this case, whole IconImage is loaded.