feat: Add support for overriding the MessageDialog style

src/Uno.UI/WinRT/Extensions/UI/Popups/MessageDialogContentDialog.cs

@@ -23,10 +23,12 @@ public MessageDialogContentDialog(MessageDialog messageDialog)
 		DefaultStyleKey = typeof(ContentDialog);
 		_messageDialog = messageDialog ?? throw new ArgumentNullException(nameof(messageDialog));
 
-		// WinUI provides a modern style for ContentDialog, which is not applied automatically - force it
-		if (Application.Current.Resources.TryGetValue("DefaultContentDialogStyle", out var resource) && resource is Style winUIStyle)
-		{
-			Style = winUIStyle;
+		var styleOverriden = TryApplyStyle(WinRTFeatureConfiguration.MessageDialog.StyleOverride);
+		if (!styleOverriden)
+        {
+			// WinUI provides a modern style for ContentDialog, which is not applied automatically.
+			// Force apply it if available.
+			TryApplyStyle("DefaultContentDialogStyle");
 		}
 
 		_commands = _messageDialog.Commands.ToList();
@@ -49,6 +51,19 @@ public MessageDialogContentDialog(MessageDialog messageDialog)
 		DefaultButton = (ContentDialogButton)(_messageDialog.DefaultCommandIndex + 1); // ContentDialogButton indexed from 1
 	}
 
+	private bool TryApplyStyle(string resourceKey)
+	{
+		if (!string.IsNullOrEmpty(resourceKey) &&
+			Application.Current.Resources.TryGetValue(resourceKey, out var resource) &&
+			resource is Style style)
+		{
+			Style = style;
+			return true;
+		}
+
+		return false;
+	}
+
 	public async Task<IUICommand> ShowAsync(CancellationToken ct)
 	{
 		var result = await base.ShowAsync().AsTask(ct);

src/Uno.UWP/UI/Popups/MessageDialog.cs

@@ -103,7 +103,7 @@ public IAsyncOperation<IUICommand> ShowAsync()
 
 	private async Task<IUICommand> ShowInnerAsync(CancellationToken ct)
 	{
-#if __IOS__ || __MACOS__ || __ANDROID__
+#if __IOS__ || __MACOS__ || __ANDROID__ || __WASM__
 		if (WinRTFeatureConfiguration.MessageDialog.UseNativeDialog)
 		{
 			return await ShowNativeAsync(ct);

src/Uno.UWP/WinRTFeatureConfiguration.MessageDialog.cs

@@ -17,5 +17,12 @@ public static class MessageDialog
 			#else
 				false;
 			#endif
+
+		/// <summary>
+		/// Allows overriding the style used by the ContentDialog
+		/// which displays the MessageDialog. Should be set to a name (Key)
+		/// of a Application-level ContentDialog style resource.
+		/// </summary>
+		public static string StyleOverride { get; set; }
 	}
 }