Doing dark/light mode resources in WPF

App.xaml

<Application x:Class="MyWPFStuff.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:MyWPFStuff"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <local:ThemeResourceDictionary >

                    <local:ThemeResourceDictionary.DarkModeResources>
                        <SolidColorBrush x:Key="Background" Color="Black"/>
                    </local:ThemeResourceDictionary.DarkModeResources>

                    <local:ThemeResourceDictionary.LightModeResources>
                        <SolidColorBrush x:Key="Background" Color="White"/>
                    </local:ThemeResourceDictionary.LightModeResources>

                  <SolidColorBrush x:Key="AlwaysWhite" Color="White"/>

                </local:ThemeResourceDictionary>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

ThemeResourceDictionary.cs

using System;
using System.Windows;
using System.Windows.Threading;

namespace MyWPFStuff
{
    public class ThemeResourceDictionary : ResourceDictionary
    {
        private readonly WeakEventListener<ThemeResourceDictionary, object, object, Microsoft.Win32.UserPreferenceChangingEventArgs> userPreferenceHandler;
        private readonly Dispatcher _CurrentDispatcher;

        /// <summary>
        /// Initializes a new instance of the <see cref="ThemeResourceDictionary"/> class.
        /// </summary>
        public ThemeResourceDictionary()
        {
            userPreferenceHandler = new WeakEventListener<ThemeResourceDictionary, object, object, Microsoft.Win32.UserPreferenceChangingEventArgs>(this, null);
            userPreferenceHandler.OnEventAction = (instance, sender, e) => SystemEvents_UserPreferenceChanging(e);
            userPreferenceHandler.OnDetachAction = (instance, source, weakListener) => Microsoft.Win32.SystemEvents.UserPreferenceChanging -= userPreferenceHandler.OnEvent;
            Microsoft.Win32.SystemEvents.UserPreferenceChanging += userPreferenceHandler.OnEvent;
            _CurrentDispatcher = Dispatcher.CurrentDispatcher;
            UpdateResources();
        }

        private void SystemEvents_UserPreferenceChanging(Microsoft.Win32.UserPreferenceChangingEventArgs e)
        {
            if (e.Category == Microsoft.Win32.UserPreferenceCategory.General && m_Theme == AppTheme.Default &&
                _CurrentDispatcher != null && !_CurrentDispatcher.HasShutdownStarted && !_CurrentDispatcher.HasShutdownFinished)
            {
                _CurrentDispatcher.BeginInvoke(new Action(() => UpdateResources()));
            }
        }

        private AppTheme m_Theme;

        /// <summary>
        /// Gets or sets the theme for the resources.
        /// </summary>
        public AppTheme Theme
        {
            get { return m_Theme; }
            set
            {
                if (m_Theme != value)
                {
                    m_Theme = value;
                    UpdateResources();
                }
            }
        }

        private void UpdateResources()
        {
            bool isDark = m_Theme == AppTheme.Dark;
            if(m_Theme == AppTheme.Default)
            {
                try
                {
                    isDark = (int?)Microsoft.Win32.Registry.GetValue("HKEY_CURRENT_USER\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize", "AppsUseLightTheme", -1) != 1;
                }
                catch { }
            }
            if (isDark)
            {
                if(this.MergedDictionaries.Contains(LightModeResources))
                    this.MergedDictionaries.Remove(LightModeResources);
                this.MergedDictionaries.Add(DarkModeResources);
            }
            else
            {
                if (this.MergedDictionaries.Contains(DarkModeResources))
                    this.MergedDictionaries.Remove(DarkModeResources);
                this.MergedDictionaries.Add(LightModeResources);
            }
        }

        public ResourceDictionary LightModeResources { get; } = new ResourceDictionary();

        public ResourceDictionary DarkModeResources { get; } = new ResourceDictionary();

    }

    /// <summary>
    /// Theme for the application
    /// </summary>
    /// <seealso cref="CalciteResources.Theme"/>
    public enum AppTheme
    {
        /// <summary>
        /// Defaults to the system theme
        /// </summary>
        Default,
        /// <summary>
        /// Light theme
        /// </summary>
        Light,
        /// <summary>
        /// Dark theme
        /// </summary>
        Dark
    }
}