feat: add headless test.

tests/HeadlessTest.Ursa/Controls/DialogTests/StyleClassTests/StyleClassTests.cs

@@ -0,0 +1,30 @@
+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using Avalonia.VisualTree;
+using Ursa.Controls;
+
+namespace HeadlessTest.Ursa.Controls.DialogTests.StyleClassTests;
+
+public class StyleClassTests
+{
+    [AvaloniaFact]
+    public void StyleClass_Changes_Button_Content()
+    {
+        var vm = new TestViewModel();
+        var ursaWindow = new TestWindow()
+        {
+            DataContext = vm,
+        };
+        ursaWindow.Show();
+        vm.InvokeDialog();
+        var dialog = ursaWindow.GetVisualDescendants().OfType<DefaultDialogControl>().SingleOrDefault();
+        Assert.NotNull(dialog);
+        Assert.Contains("Custom", dialog.Classes);
+        var okButton = dialog.GetVisualDescendants().OfType<Button>().SingleOrDefault(a=>a.Name == DefaultDialogControl.PART_OKButton);
+        Assert.NotNull(okButton);
+        Assert.Equal("CUSTOM", okButton.Content);
+        var cancelButton = dialog.GetVisualDescendants().OfType<Button>().SingleOrDefault(a=>a.Name == DefaultDialogControl.PART_CancelButton);
+        Assert.NotNull(cancelButton);
+        Assert.Equal("取消", cancelButton.Content);
+    }
+}