Enable opting out of system theme color changes

Author: dipeshmsft

src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Win32/UxThemeWrapper.cs

@@ -275,9 +275,9 @@ private static void GetThemeNameAndColor(out string themeName, out string themeC
                     themeName = "Aero2";
                 }
 
+                NameValueCollection appSettings = null;
 #if DEBUG
                 // for debugging, config file can override the theme name
-                NameValueCollection appSettings = null;
                 try
                 {
                     appSettings = ConfigurationManager.AppSettings;
@@ -302,6 +302,32 @@ private static void GetThemeNameAndColor(out string themeName, out string themeC
                 {
                     themeName = "Fluent";
                     themeColor = ThemeManager.IsSystemThemeLight() ? "Light" : "Dark";
+
+                    appSettings = null;
+                    try
+                    {
+                        appSettings = ConfigurationManager.AppSettings;
+                    }
+                    catch (ConfigurationErrorsException)
+                    {
+                    }
+
+                    if (appSettings != null)
+                    {
+                        string tc = appSettings["ThemeColorOverride"];
+                        if (!String.IsNullOrEmpty(tc))
+                        {
+                            switch(tc.ToLowerInvariant())
+                            {
+                                case "light":
+                                case "dark":
+                                    themeColor = tc;
+                                    ThemeManager.OverrideThemeColor(themeColor);
+                                    break;
+                            }
+                            themeColor = tc;
+                        }
+                    }
                 }
             }
             else

src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/ThemeManager.cs

@@ -102,7 +102,14 @@ internal static void UpdateBackdropAndImmersiveMode(IEnumerable windows = null)
                 continue;
             }
 
-            SetImmersiveDarkMode(window, !ThemeManager.IsSystemThemeLight());
+            if (ThemeManager.UseCustomThemeColor)
+            {
+                SetImmersiveDarkMode(window, ThemeManager.RequestedThemeColor == "Dark");
+            }
+            else
+            {
+                SetImmersiveDarkMode(window, !ThemeManager.IsSystemThemeLight());
+            }
             WindowBackdropManager.SetBackdrop(window, SystemParameters.HighContrast ? WindowBackdropType.None : WindowBackdropType.MainWindow);
         }
     }
@@ -207,6 +214,27 @@ internal static bool IsSystemThemeLight()
         return useLightTheme != null && useLightTheme != 0;
     }
 
+
+    internal static void OverrideThemeColor(string requestedThemeColor)
+    {
+        if (requestedThemeColor == null)
+        {
+            return;
+        }
+
+        _requestedThemeColor = "System";
+        if (requestedThemeColor == "Light" || requestedThemeColor == "Dark")
+        {
+            _requestedThemeColor = requestedThemeColor;
+        }
+
+        if (IsFluentThemeEnabled)
+        {
+            var themeColorResourceUri = GetFluentWindowThemeColorResourceUri(_currentApplicationTheme, _currentUseLightMode);
+            AddOrUpdateThemeResources(themeColorResourceUri);
+        }
+    }
+
     /// <summary>
     ///  Update the Fluent theme resources with the values in new dictionary.
     /// </summary>
@@ -241,6 +269,10 @@ private static void AddOrUpdateThemeResources(Uri dictionaryUri)
     // TODO : Find a better way to deal with different default font sizes for different themes.
     internal static double DefaultFluentThemeFontSize => 14;
 
+    internal static bool UseCustomThemeColor => _requestedThemeColor != "System";
+
+    internal static string RequestedThemeColor => _requestedThemeColor;
+
     #endregion
 
     #region Private Methods
@@ -279,5 +311,7 @@ string s when s.Contains("hc1") => "hc1.xaml",
 
     private static bool _isFluentThemeInitialized = false;
 
+    private static string _requestedThemeColor = "System";
+
     #endregion
 }