如何正确保存节目主题？

我想实现更改程序的主题并保存它。当您退出程序时，将加载保存的主题。当您选择主题时，主题会发生变化，但不会保存（需要修复什么？

项目浏览器

应用程序.xaml

<Application x:Class="ConvertToImage.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:ConvertToImage"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Styles\DarkTheme.xaml"/>
                <ResourceDictionary Source="Styles\LightTheme.xaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>
</Application>

应用程序.xaml.cs

using System.Windows;
using ConvertToImage.Properties; 

namespace ConvertToImage
{
    /// <summary>
    /// Interaction logic for App.xaml
    /// </summary>
    public partial class App : Application
    {
        private void Application_Startup(object sender, StartupEventArgs e)
        {
            string savedTheme = Settings.Default.ThemeName;
            ApplyTheme(savedTheme);
        }

        public static void ApplyTheme(string theme)
        {
            ResourceDictionary resourceDict = new ();
            switch (theme)
            {
                case "Dark":
                    resourceDict.Source = new Uri("/Styles/DarkTheme.xaml", UriKind.Relative);
                    break;
                case "Light":
                    resourceDict.Source = new Uri("/Styles/LightTheme.xaml", UriKind.Relative);
                    break;
            }
            Application.Current.Resources.MergedDictionaries.Clear();
            Application.Current.Resources.MergedDictionaries.Add(resourceDict);
        }
    }
}

文件设置.Designer.cs

//------------------------------------------------------------------------------
// <auto-generated>
//     Этот код создан программой.
//     Исполняемая версия:4.0.30319.42000
//
//     Изменения в этом файле могут привести к неправильной работе и будут потеряны в случае
//     повторной генерации кода.
// </auto-generated>
//------------------------------------------------------------------------------

namespace ConvertToImage.Properties {
    
    
    [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.VisualStudio.Editors.SettingsDesigner.SettingsSingleFileGenerator", "17.10.0.0")]
    public  partial class Settings : global::System.Configuration.ApplicationSettingsBase {
        
        public static Settings defaultInstance = ((Settings)(global::System.Configuration.ApplicationSettingsBase.Synchronized(new Settings())));
        
        public static Settings Default {
            get {
                return defaultInstance;
            }
        }
        
        [global::System.Configuration.UserScopedSettingAttribute()]
        [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
        [global::System.Configuration.DefaultSettingValueAttribute("")]
        public string ThemeName {
            get {
                return ((string)(this["ThemeName"]));
            }
            set {
                this["ThemeName"] = value;
            }
        }
    }
}

带按钮的页面

<Grid Style="{DynamicResource GridBackgroundBrush}">

     <StackPanel HorizontalAlignment="Center" VerticalAlignment="Center">
         <Button Width="200" Height="30" Content="Светлый" 
                  x:Name="LinghtBtn" Click="Test_Click" />

         <Button Width="200" x:Name="DartBtn" Content="Тёмный" 
     Height="30" Click="Test_Click" />
     </StackPanel>
 </Grid>

切换主题

 private void Test_Click(object sender, RoutedEventArgs e)
 {
     Button clickedButton = sender as Button;
     if (clickedButton != null)
     {
         string selectedTheme = clickedButton.Name == "LinghtBtn" ? "Light" : "Dark";              
         Properties.Settings.Default.ThemeName = selectedTheme;
         Properties.Settings.Default.Save();                
         App.ApplyTheme(selectedTheme);
     }
 }

我期望主题被更改并保存，但主题没有保存。