fix: Improve system dark theme detection on Linux

src/Uno.UI.Runtime.Skia.Gtk/Helpers/Theming/GtkSystemThemeHelperExtension.cs

@@ -11,9 +11,11 @@ namespace Uno.UI.Runtime.Skia.Gtk.Extensions.Helpers.Theming
 	internal class GtkSystemThemeHelperExtension : ISystemThemeHelperExtension, IDisposable
 	{
 		private bool _disposedValue;
+		private string _previousThemeName;
 
 		internal GtkSystemThemeHelperExtension(object owner)
 		{
+			_previousThemeName = Settings.Default.ThemeName;
 			ObserveSystemTheme();
 		}
 
@@ -29,6 +31,7 @@ private void ObserveSystemTheme()
 			{
 				var settings = Settings.Default;
 				settings.AddNotification(nameof(settings.ApplicationPreferDarkTheme), ApplicationPreferDarkThemeHandler);
+				settings.AddNotification(nameof(settings.ThemeName), ApplicationPreferDarkThemeHandler);
 			}
 		}
 
@@ -42,11 +45,24 @@ private void UnobserveSystemTheme()
 			{
 				var settings = Settings.Default;
 				settings.RemoveNotification(nameof(settings.ApplicationPreferDarkTheme), ApplicationPreferDarkThemeHandler);
+				settings.RemoveNotification(nameof(settings.ThemeName), ApplicationPreferDarkThemeHandler);
 			}
 		}
 
 		private void ApplicationPreferDarkThemeHandler(object o, GLib.NotifyArgs args)
-			=> SystemThemeChanged?.Invoke(o, EventArgs.Empty);
+		{
+			var settings = Settings.Default;
+			if (args.Property == nameof(settings.ApplicationPreferDarkTheme) || IsGtkThemeDark(_previousThemeName) != IsGtkThemeDark(settings.ThemeName))
+			{
+				SystemThemeChanged?.Invoke(o, EventArgs.Empty);
+			}
+			_previousThemeName = settings.ThemeName;
+		}
+
+		private bool IsGtkThemeDark(string themeName)
+		{
+			return themeName.Contains("-dark", StringComparison.OrdinalIgnoreCase);
+		}
 
 		public SystemTheme GetSystemTheme()
 		{
@@ -56,7 +72,7 @@ public SystemTheme GetSystemTheme()
 			}
 			else
 			{
-				return Settings.Default.ApplicationPreferDarkTheme ? SystemTheme.Dark : SystemTheme.Light;
+				return (Settings.Default.ApplicationPreferDarkTheme || IsGtkThemeDark(Settings.Default.ThemeName)) ? SystemTheme.Dark : SystemTheme.Light;
 			}
 		}