Enable dark mode if the system is in dark mode for Win32 windows

Flow.Launcher.Infrastructure/Win32Helper.cs

@@ -40,7 +40,7 @@
         {
             var cloaked = cloak ? 1 : 0;
 
             return PInvoke.DwmSetWindowAttribute(
                 GetWindowHandle(window),
                 DWMWINDOWATTRIBUTE.DWMWA_CLOAK,
                 &cloaked,
@@ -52,14 +52,14 @@
             var backdropType = backdrop switch
             {
                 BackdropTypes.Acrylic => DWM_SYSTEMBACKDROP_TYPE.DWMSBT_TRANSIENTWINDOW,
                 BackdropTypes.Mica => DWM_SYSTEMBACKDROP_TYPE.DWMSBT_MAINWINDOW,
                 BackdropTypes.MicaAlt => DWM_SYSTEMBACKDROP_TYPE.DWMSBT_TABBEDWINDOW,
                 _ => DWM_SYSTEMBACKDROP_TYPE.DWMSBT_AUTO
             };
 
             return PInvoke.DwmSetWindowAttribute(
                 GetWindowHandle(window),
                 DWMWINDOWATTRIBUTE.DWMWA_SYSTEMBACKDROP_TYPE,
                 &backdropType,
                 (uint)Marshal.SizeOf<int>()).Succeeded;
         }
@@ -791,5 +791,41 @@
         }
 
         #endregion
+
+        #region Win32 Dark Mode
+
+        /*
+         * Inspired by https://github.com/ysc3839/win32-darkmode
+         */
+
+        [DllImport("uxtheme.dll", EntryPoint = "#135", SetLastError = true)]
+        private static extern int SetPreferredAppMode(int appMode);
+
+        public static void EnableWin32DarkMode(string colorScheme)
+        {
+            try
+            {
+                // Undocumented API from Windows 10 1809
+                if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows) &&
+                    Environment.OSVersion.Version.Build >= 17763)
+                {
+                    var flag = colorScheme switch
+                    {
+                        Constant.Light => 3, // ForceLight
+                        Constant.Dark => 2, // ForceDark
+                        Constant.System => 1, // AllowDark
+                        _ => 0 // Default
+                    };
+                    _ = SetPreferredAppMode(flag);
+                }
+
+            }
+            catch
+            {
+                // Ignore errors on unsupported OS
+            }
+        }
+
+        #endregion
     }
 }

Flow.Launcher/App.xaml.cs

@@ -188,6 +188,9 @@ await API.StopwatchLogInfoAsync(ClassName, "Startup cost", async () =>
 
                 Notification.Install();
 
+                // Enable Win32 dark mode if the system is in dark mode before creating all windows
+                Win32Helper.EnableWin32DarkMode(_settings.ColorScheme);
+
                 Ioc.Default.GetRequiredService<Portable>().PreStartCleanUpAfterPortabilityUpdate();
 
                 API.LogInfo(ClassName, "Begin Flow Launcher startup ----------------------------------------------------");

Flow.Launcher/SettingPages/ViewModels/SettingsPaneThemeViewModel.cs

@@ -136,6 +136,7 @@ public string ColorScheme
             };
             Settings.ColorScheme = value;
             _ = _theme.RefreshFrameAsync();
+            Win32Helper.EnableWin32DarkMode(value);
         }
     }