diff --git a/src/dev/impl/DevToys.MonacoEditor/ts-helpermethods/otherScriptsToBeOrganized.ts b/src/dev/impl/DevToys.MonacoEditor/ts-helpermethods/otherScriptsToBeOrganized.ts
index 47f5a9b01e..365bd6e8a1 100644
--- a/src/dev/impl/DevToys.MonacoEditor/ts-helpermethods/otherScriptsToBeOrganized.ts
+++ b/src/dev/impl/DevToys.MonacoEditor/ts-helpermethods/otherScriptsToBeOrganized.ts
@@ -115,7 +115,7 @@ var updateOptions = function (opt: monaco.editor.IEditorOptions) {
 
 var updateDiffOptions = function (opt: monaco.editor.IDiffEditorOptions) {
     var diffEditor = (editor as unknown) as monaco.editor.IStandaloneDiffEditor;
-    if (opt != null && typeof opt === "object") {
+    if (diffEditor != null && opt != null && typeof opt === "object") {
         diffEditor.updateOptions(opt);
     }
 };
diff --git a/src/dev/impl/DevToys/Api/Core/Theme/IThemeListener.cs b/src/dev/impl/DevToys/Api/Core/Theme/IThemeListener.cs
index 28b04bcfee..a6ea87e98e 100644
--- a/src/dev/impl/DevToys/Api/Core/Theme/IThemeListener.cs
+++ b/src/dev/impl/DevToys/Api/Core/Theme/IThemeListener.cs
@@ -1,6 +1,7 @@
 #nullable enable
 
 using System;
+using Windows.UI.Xaml;
 
 namespace DevToys.Api.Core.Theme
 {
@@ -16,6 +17,11 @@ public interface IThemeListener
         /// </summary>
         AppTheme CurrentAppTheme { get; }
 
+        /// <summary>
+        /// Gets the actual theme applied in the app.
+        /// </summary>
+        ApplicationTheme ActualAppTheme { get; }
+
         /// <summary>
         /// Gets or sets a value indicating whether the current theme is high contrast.
         /// </summary>
diff --git a/src/dev/impl/DevToys/Assets/ReleaseNote.txt b/src/dev/impl/DevToys/Assets/ReleaseNote.txt
index 5a3a4229b2..3e5d826b15 100644
--- a/src/dev/impl/DevToys/Assets/ReleaseNote.txt
+++ b/src/dev/impl/DevToys/Assets/ReleaseNote.txt
@@ -5,5 +5,6 @@
   • RegEx Tester tool got new options.
   • New layout.
   • Text editor can now use any font.
+  • Russian translation improved.
 🐛 Bug Fixes
   • Minor bugs fixed.
\ No newline at end of file
diff --git a/src/dev/impl/DevToys/Core/Theme/ThemeListener.cs b/src/dev/impl/DevToys/Core/Theme/ThemeListener.cs
index 55ee10e8ea..bffebdb22a 100644
--- a/src/dev/impl/DevToys/Core/Theme/ThemeListener.cs
+++ b/src/dev/impl/DevToys/Core/Theme/ThemeListener.cs
@@ -23,6 +23,8 @@ internal sealed class ThemeListener : IThemeListener
 
         public AppTheme CurrentAppTheme => _settingsProvider.GetSetting(PredefinedSettings.Theme);
 
+        public ApplicationTheme ActualAppTheme { get; private set; }
+
         public bool IsHighContrast { get; private set; }
 
         public event EventHandler? ThemeChanged;
@@ -59,6 +61,7 @@ public void ApplyDesiredColorTheme()
             if (Window.Current.Content is FrameworkElement frameworkElement)
             {
                 frameworkElement.RequestedTheme = theme == AppTheme.Light ? ElementTheme.Light : ElementTheme.Dark;
+                ActualAppTheme = theme == AppTheme.Light ? ApplicationTheme.Light : ApplicationTheme.Dark;
             }
         }
 
@@ -96,6 +99,7 @@ private void SettingsProvider_SettingChanged(object sender, SettingChangedEventA
             if (string.Equals(PredefinedSettings.Theme.Name, e.SettingName, StringComparison.Ordinal))
             {
                 ApplyDesiredColorTheme();
+                ThemeChanged?.Invoke(this, EventArgs.Empty);
             }
         }
 
diff --git a/src/dev/impl/DevToys/UI/Controls/ExpandableSettingControl.xaml.cs b/src/dev/impl/DevToys/UI/Controls/ExpandableSettingControl.xaml.cs
index 34adeba794..4e6053d1d9 100644
--- a/src/dev/impl/DevToys/UI/Controls/ExpandableSettingControl.xaml.cs
+++ b/src/dev/impl/DevToys/UI/Controls/ExpandableSettingControl.xaml.cs
@@ -1,5 +1,7 @@
 #nullable enable
 
+using DevToys.Api.Core.Theme;
+using DevToys.Core;
 using Windows.UI.Xaml;
 using Windows.UI.Xaml.Controls;
 using Windows.UI.Xaml.Markup;
@@ -10,9 +12,13 @@ namespace DevToys.UI.Controls
     [ContentProperty(Name = nameof(SettingActionableElement))]
     public sealed partial class ExpandableSettingControl : UserControl
     {
+        private bool _isAppThemeChanging;
+        private bool _isDefaultHeaderBackgroundValue = true;
+        private bool _isDefaultContentBackgroundValue = true;
+
         public FrameworkElement? SettingActionableElement { get; set; }
 
-        public static readonly DependencyProperty TitleProperty 
+        public static readonly DependencyProperty TitleProperty
             = DependencyProperty.Register(
                 nameof(Title),
                 typeof(string),
@@ -51,7 +57,7 @@ public IconElement Icon
             set => SetValue(IconProperty, value);
         }
 
-        public static readonly DependencyProperty ExpandableContentProperty 
+        public static readonly DependencyProperty ExpandableContentProperty
             = DependencyProperty.Register(
                 nameof(ExpandableContent),
                 typeof(FrameworkElement),
@@ -82,7 +88,14 @@ public static readonly DependencyProperty ContentBackgroundProperty
                 nameof(ContentBackground),
                 typeof(Brush),
                 typeof(ExpandableSettingControl),
-                new PropertyMetadata(Application.Current.Resources["ExpanderContentBackground"]));
+                new PropertyMetadata(null, (d, e) =>
+                {
+                    var ctrl = (ExpandableSettingControl)d;
+                    if (!ctrl._isAppThemeChanging)
+                    {
+                        ctrl._isDefaultContentBackgroundValue = false;
+                    }
+                }));
 
         public Brush ContentBackground
         {
@@ -95,7 +108,14 @@ public static readonly DependencyProperty HeaderBackgroundProperty
                 nameof(HeaderBackground),
                 typeof(Brush),
                 typeof(ExpandableSettingControl),
-                new PropertyMetadata(Application.Current.Resources["ExpanderHeaderBackground"]));
+                new PropertyMetadata(null, (d, e) =>
+                {
+                    var ctrl = (ExpandableSettingControl)d;
+                    if (!ctrl._isAppThemeChanging)
+                    {
+                        ctrl._isDefaultHeaderBackgroundValue = false;
+                    }
+                }));
 
         public Brush HeaderBackground
         {
@@ -106,6 +126,30 @@ public Brush HeaderBackground
         public ExpandableSettingControl()
         {
             InitializeComponent();
+
+            MefComposer.Provider.Import<IThemeListener>().ThemeChanged += ExpandableSettingControl_ThemeChanged;
+
+            _isAppThemeChanging = true;
+            ContentBackground = (Brush)Application.Current.Resources["ExpanderContentBackground"];
+            HeaderBackground = (Brush)Application.Current.Resources["ExpanderHeaderBackground"];
+            _isAppThemeChanging = false;
+        }
+
+        private void ExpandableSettingControl_ThemeChanged(object sender, System.EventArgs e)
+        {
+            _isAppThemeChanging = true;
+
+            if (_isDefaultContentBackgroundValue)
+            {
+                ContentBackground = (Brush)Application.Current.Resources["ExpanderContentBackground"];
+            }
+
+            if (_isDefaultHeaderBackgroundValue)
+            {
+                HeaderBackground = (Brush)Application.Current.Resources["ExpanderHeaderBackground"];
+            }
+
+            _isAppThemeChanging = false;
         }
     }
 }
diff --git a/src/dev/impl/DevToys/ViewModels/Tools/MarkdownPreview/MarkdownPreviewToolViewModel.cs b/src/dev/impl/DevToys/ViewModels/Tools/MarkdownPreview/MarkdownPreviewToolViewModel.cs
index 7ca8f09c21..a51848dc04 100644
--- a/src/dev/impl/DevToys/ViewModels/Tools/MarkdownPreview/MarkdownPreviewToolViewModel.cs
+++ b/src/dev/impl/DevToys/ViewModels/Tools/MarkdownPreview/MarkdownPreviewToolViewModel.cs
@@ -1,6 +1,7 @@
 #nullable enable
 
 using DevToys.Api.Core.Settings;
+using DevToys.Api.Core.Theme;
 using DevToys.Api.Tools;
 using DevToys.Core;
 using DevToys.Core.Threading;
@@ -23,12 +24,13 @@ public sealed class MarkdownPreviewToolViewModel : ObservableRecipient, IToolVie
     {
         private static string? _htmlDocument;
 
-        private static readonly SettingDefinition<ApplicationTheme> ThemeSetting
+        private static readonly SettingDefinition<string?> ThemeSetting
             = new(
                 name: $"{nameof(MarkdownPreviewToolViewModel)}.{nameof(ThemeSetting)}",
                 isRoaming: true,
-                defaultValue: ApplicationTheme.Light);
+                defaultValue: null);
 
+        private readonly IThemeListener _themeListener;
         private readonly Queue<string> _workQueue = new();
 
         private bool _workInProgress;
@@ -53,12 +55,20 @@ internal string? InputValue
 
         internal ApplicationTheme Theme
         {
-            get => SettingsProvider.GetSetting(ThemeSetting);
+            get
+            {
+                string? theme = SettingsProvider.GetSetting(ThemeSetting);
+                if (string.IsNullOrEmpty(theme))
+                {
+                    theme = _themeListener.ActualAppTheme.ToString();
+                }
+                return (ApplicationTheme)Enum.Parse(typeof(ApplicationTheme), theme);
+            }
             set
             {
-                if (SettingsProvider.GetSetting(ThemeSetting) != value)
+                if (!string.Equals(SettingsProvider.GetSetting(ThemeSetting), value.ToString()))
                 {
-                    SettingsProvider.SetSetting(ThemeSetting, value);
+                    SettingsProvider.SetSetting(ThemeSetting, value.ToString());
                     OnPropertyChanged();
                     QueueWork();
                 }
@@ -68,8 +78,9 @@ internal ApplicationTheme Theme
         internal ISettingsProvider SettingsProvider { get; }
 
         [ImportingConstructor]
-        public MarkdownPreviewToolViewModel(ISettingsProvider settingsProvider)
+        public MarkdownPreviewToolViewModel(ISettingsProvider settingsProvider, IThemeListener themeListener)
         {
+            _themeListener = themeListener;
             SettingsProvider = settingsProvider;
 
             // Activate the view model's messenger.
diff --git a/tools/app-version-number.txt b/tools/app-version-number.txt
index 1a09779c6a..c05a067712 100644
--- a/tools/app-version-number.txt
+++ b/tools/app-version-number.txt
@@ -1 +1 @@
-0.6.5.0
\ No newline at end of file
+0.6.5.1
\ No newline at end of file