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);
+    }
+}

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

@@ -0,0 +1,17 @@
+using Avalonia.Controls;
+using CommunityToolkit.Mvvm.ComponentModel;
+using Ursa.Controls;
+
+namespace HeadlessTest.Ursa.Controls.DialogTests.StyleClassTests;
+
+public class TestViewModel: ObservableObject
+{
+    public void InvokeDialog()
+    {
+        OverlayDialog.Show<TextBlock, string>("Hello World", options: new OverlayDialogOptions()
+        {
+            Buttons = DialogButton.OKCancel,
+            StyleClass = "Custom",
+        });
+    }
+}

tests/HeadlessTest.Ursa/Controls/DialogTests/StyleClassTests/TestWindow.axaml

@@ -0,0 +1,19 @@
+<u:UrsaWindow xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:u="https://irihi.tech/ursa"
+        mc:Ignorable="d" d:DesignWidth="800"
+        d:DesignHeight="450"
+        x:Class="HeadlessTest.Ursa.Controls.DialogTests.StyleClassTests.TestWindow"
+        Title="TestWindow">
+    <u:UrsaWindow.Styles>
+        <Style Selector="u|DefaultDialogControl.Custom">
+            <Style Selector="^ /template/ Button#PART_OKButton">
+                <Setter Property="Content" Value="CUSTOM"/>
+            </Style>
+        </Style>
+    </u:UrsaWindow.Styles>
+    Welcome to Avalonia!
+
+</u:UrsaWindow>

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

@@ -0,0 +1,14 @@
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Markup.Xaml;
+using Ursa.Controls;
+
+namespace HeadlessTest.Ursa.Controls.DialogTests.StyleClassTests;
+
+public partial class TestWindow : UrsaWindow
+{
+    public TestWindow()
+    {
+        InitializeComponent();
+    }
+}