[Settings]Theme override fix and cleanup (#32362)

* theme override fix and cleanup

* test fix

src/settings-ui/Settings.UI/Helpers/WindowHelper.cs

@@ -0,0 +1,58 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.IO;
+using System.Runtime.InteropServices;
+using System.Text.Json;
+using Microsoft.UI.Xaml;
+
+namespace Microsoft.PowerToys.Settings.UI.Helpers
+{
+    internal sealed class WindowHelper
+    {
+        private static string _placementPath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData), @"Microsoft\PowerToys\settings-placement.json");
+
+        public static WINDOWPLACEMENT DeserializePlacementOrDefault(IntPtr handle)
+        {
+            try
+            {
+                var json = File.ReadAllText(_placementPath);
+                var placement = JsonSerializer.Deserialize<WINDOWPLACEMENT>(json);
+
+                placement.Length = Marshal.SizeOf(typeof(WINDOWPLACEMENT));
+                placement.Flags = 0;
+                placement.ShowCmd = (placement.ShowCmd == NativeMethods.SW_SHOWMAXIMIZED) ? NativeMethods.SW_SHOWMAXIMIZED : NativeMethods.SW_SHOWNORMAL;
+                return placement;
+            }
+            catch (Exception)
+            {
+            }
+
+            _ = NativeMethods.GetWindowPlacement(handle, out var defaultPlacement);
+            return defaultPlacement;
+        }
+
+        public static void SerializePlacement(IntPtr handle)
+        {
+            _ = NativeMethods.GetWindowPlacement(handle, out var placement);
+            try
+            {
+                var json = JsonSerializer.Serialize(placement);
+                File.WriteAllText(_placementPath, json);
+            }
+            catch (Exception)
+            {
+            }
+        }
+
+        public static void SetTheme(Window window, ElementTheme theme)
+        {
+            if (window.Content is FrameworkElement rootElement)
+            {
+                rootElement.RequestedTheme = theme;
+            }
+        }
+    }
+}