Refactor System-handling for SetColor.

Author: KlausLoeffelmann

src/System.Windows.Forms/src/System/Windows/Forms/Application.cs

@@ -50,7 +50,7 @@ public sealed partial class Application
 
     private const string DarkModeKeyPath = "HKEY_CURRENT_USER\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\Themes\\Personalize";
     private const string DarkModeKey = "AppsUseLightTheme";
-    private const int DarkModeNotAvailable = -1;
+    private const int SystemDarkModeDisabled = 1;
 
     /// <summary>
     ///  Events the user can hook into
@@ -283,6 +283,12 @@ public static void SetColorMode(SystemColorMode systemColorMode)
                 return;
             }
 
+            if (GetSystemColorModeInternal() != SystemDarkModeDisabled)
+            {
+                s_systemColorMode = SystemColorMode.Dark;
+                return;
+            }
+
             s_systemColorMode = SystemColorMode.Classic;
         }
         finally
@@ -358,20 +364,16 @@ private static int GetSystemColorModeInternal()
 
         int systemColorMode = DarkModeNotAvailable;
 
-        // Dark mode is supported when we are >= W11/22000
-        // Technically, we could go earlier, but then the APIs we're using weren't officially public.
-        if (OsVersion.IsWindows11_OrGreater())
+        try
+        {
+            // 0 for dark mode and |1| for light mode.
+            systemColorMode = Math.Abs((Registry.GetValue(
+                keyName: DarkModeKeyPath,
+                valueName: DarkModeKey,
+                defaultValue: SystemDarkModeDisabled) as int?) ?? systemColorMode);
+        }
+        catch (Exception ex) when (!ex.IsCriticalException())
         {
-            try
-            {
-                systemColorMode = (Registry.GetValue(
-                    keyName: DarkModeKeyPath,
-                    valueName: DarkModeKey,
-                    defaultValue: DarkModeNotAvailable) as int?) ?? systemColorMode;
-            }
-            catch (Exception ex) when (!ex.IsCriticalException())
-            {
-            }
         }
 
         return systemColorMode;