Add accessibility name to theme toggle button and helper method for visual tree search

- Added MainWindow_Loaded event handler to set the automation name of the theme toggle button to the localized "MoreOptions" string using ServiceLib.Resx.ResUI.
- Introduced FindVisualChild helper method to recursively search for a named child element in the visual tree.
- Included necessary using directives for System.Windows.Automation and System.Windows.Controls.Primitives.

v2rayN/v2rayN/Views/MainWindow.xaml.cs

@@ -1,7 +1,9 @@
 using System.ComponentModel;
 using System.Reactive.Disposables;
 using System.Windows;
+using System.Windows.Automation;
 using System.Windows.Controls;
+using System.Windows.Controls.Primitives;
 using System.Windows.Input;
 using System.Windows.Interop;
 using System.Windows.Media;
@@ -23,6 +25,8 @@ public partial class MainWindow
     {
         InitializeComponent();
 
+        Loaded += MainWindow_Loaded;
+
         _config = AppHandler.Instance.Config;
         ThreadPool.RegisterWaitForSingleObject(App.ProgramStarted, OnProgramStarted, null, -1, false);
 
@@ -155,6 +159,15 @@ public partial class MainWindow
         }));
     }
 
+    private void MainWindow_Loaded(object sender, RoutedEventArgs e)
+    {
+                var popupBox = themePopupBox;
+
+        var toggleButton = FindVisualChild<ToggleButton>(popupBox, "PART_Toggle");
+
+        AutomationProperties.SetName(toggleButton, ServiceLib.Resx.ResUI.MoreOptions);
+    }
+
     private void DelegateSnackMsg(string content)
     {
         Application.Current?.Dispatcher.Invoke((() =>
@@ -466,5 +479,26 @@ public partial class MainWindow
         }
     }
 
+    private T FindVisualChild<T>(DependencyObject parent, string name) where T : DependencyObject
+    {
+        if (parent == null)
+            return null;
+
+        for (int i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
+        {
+            var child = VisualTreeHelper.GetChild(parent, i);
+            if (child is T childType && child is FrameworkElement element && element.Name == name)
+            {
+                return childType;
+            }
+
+            var result = FindVisualChild<T>(child, name);
+            if (result != null)
+                return result;
+        }
+
+        return null;
+    }
+
     #endregion UI
 }