Alternate approach to #2947

ILSpy/ExtensionMethods.cs

@@ -161,5 +161,10 @@ namespace ICSharpCode.ILSpy
 				container.IsSelected = true;
 			view.Focus();
 		}
+
+		public static double ToGray(this Color? color)
+		{
+			return color?.R * 0.3 + color?.G * 0.6 + color?.B * 0.1 ?? 0.0;
+		}
 	}
 }

ILSpy/NativeMethods.cs

@@ -193,6 +193,16 @@ namespace ICSharpCode.ILSpy
 				return null;
 			}
 		}
+
+		[DllImport("dwmapi.dll", PreserveSig = true)]
+		public static extern int DwmSetWindowAttribute(IntPtr hwnd, DwmWindowAttribute attr, ref int attrValue, int attrSize);
+
+		public static bool UseImmersiveDarkMode(IntPtr hWnd, bool enable)
+		{
+			int darkMode = enable ? 1 : 0;
+			int hr = DwmSetWindowAttribute(hWnd, DwmWindowAttribute.UseImmersiveDarkMode, ref darkMode, sizeof(int));
+			return hr >= 0;
+		}
 	}
 
 	[return: MarshalAs(UnmanagedType.Bool)]
@@ -212,4 +222,33 @@ namespace ICSharpCode.ILSpy
 			this.Buffer = buffer;
 		}
 	}
+
+	public enum DwmWindowAttribute : uint
+	{
+		NCRenderingEnabled = 1,
+		NCRenderingPolicy,
+		TransitionsForceDisabled,
+		AllowNCPaint,
+		CaptionButtonBounds,
+		NonClientRtlLayout,
+		ForceIconicRepresentation,
+		Flip3DPolicy,
+		ExtendedFrameBounds,
+		HasIconicBitmap,
+		DisallowPeek,
+		ExcludedFromPeek,
+		Cloak,
+		Cloaked,
+		FreezeRepresentation,
+		PassiveUpdateMode,
+		UseHostBackdropBrush,
+		UseImmersiveDarkMode = 20,
+		WindowCornerPreference = 33,
+		BorderColor,
+		CaptionColor,
+		TextColor,
+		VisibleFrameBorderThickness,
+		SystemBackdropType,
+		Last
+	}
 }

ILSpy/Themes/WindowStyleManagerBehavior.cs

@@ -16,11 +16,16 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+using System;
 using System.ComponentModel;
 using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Interop;
+using System.Windows.Media;
 
 using ICSharpCode.ILSpy.Options;
 
+using TomsToolbox.Essentials;
 using TomsToolbox.Wpf;
 using TomsToolbox.Wpf.Interactivity;
 
@@ -30,32 +35,46 @@ namespace ICSharpCode.ILSpy.Themes
 	{
 		private static readonly DispatcherThrottle restartNotificationThrottle = new DispatcherThrottle(ShowRestartNotification);
 
+		private INotifyChanged _foreground;
+		private INotifyChanged _background;
+
 		protected override void OnAttached()
 		{
 			base.OnAttached();
 
 			MainWindow.Instance.CurrentDisplaySettings.PropertyChanged += DisplaySettings_PropertyChanged;
 
-			UpdateWindowStyle();
+			_foreground = AssociatedObject.Track(Control.ForegroundProperty);
+			_background = AssociatedObject.Track(Control.BackgroundProperty);
 
+			_foreground.Changed += Color_Changed;
+			_background.Changed += Color_Changed;
+
+			UpdateWindowStyle();
+			ApplyThemeToWindowCaption();
 		}
 
 		protected override void OnDetaching()
 		{
 			base.OnDetaching();
 
+			_foreground.Changed -= Color_Changed;
+			_background.Changed -= Color_Changed;
+
 			MainWindow.Instance.CurrentDisplaySettings.PropertyChanged -= DisplaySettings_PropertyChanged;
 		}
 
-		private void UpdateWindowStyle()
+		private void Color_Changed(object sender, EventArgs e)
 		{
-			if (!MainWindow.Instance.CurrentDisplaySettings.StyleWindowTitleBar)
-			{
-				return;
-			}
+			ApplyThemeToWindowCaption();
+		}
 
+		private void UpdateWindowStyle()
+		{
 			var window = AssociatedObject;
-			window.Style = (Style)window.FindResource(TomsToolbox.Wpf.Styles.ResourceKeys.WindowStyle);
+
+			if (MainWindow.Instance.CurrentDisplaySettings.StyleWindowTitleBar)
+				window.Style = (Style)window.FindResource(TomsToolbox.Wpf.Styles.ResourceKeys.WindowStyle);
 		}
 
 		private static void ShowRestartNotification()
@@ -76,5 +95,32 @@ namespace ICSharpCode.ILSpy.Themes
 				UpdateWindowStyle();
 			}
 		}
+
+		private void ApplyThemeToWindowCaption()
+		{
+			var window = AssociatedObject;
+
+			IntPtr hwnd = new WindowInteropHelper(window).Handle;
+
+			if (hwnd != IntPtr.Zero)
+			{
+				var foreground = ((window.Foreground as SolidColorBrush)?.Color).ToGray();
+				var background = ((window.Background as SolidColorBrush)?.Color).ToGray();
+
+				var isDarkTheme = background < foreground;
+
+				NativeMethods.UseImmersiveDarkMode(hwnd, isDarkTheme);
+			}
+			else
+			{
+				void Initialized(object o, EventArgs eventArgs)
+				{
+					ApplyThemeToWindowCaption();
+					window.SourceInitialized -= Initialized;
+				}
+
+				window.SourceInitialized += Initialized;
+			}
+		}
 	}
 }