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

* theme override fix and cleanup

* test fix

src/settings-ui/Settings.UI/Services/ThemeService.cs

@@ -0,0 +1,48 @@
+// 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 Microsoft.PowerToys.Settings.UI.Library;
+using Microsoft.PowerToys.Settings.UI.Library.Interfaces;
+using Microsoft.UI.Xaml;
+
+namespace Microsoft.PowerToys.Settings.UI.Services
+{
+    public class ThemeService
+    {
+        private readonly ISettingsRepository<GeneralSettings> _generalSettingsRepository;
+
+        public event EventHandler<ElementTheme> ThemeChanged;
+
+        public ElementTheme Theme { get; private set; } = ElementTheme.Default;
+
+        public ThemeService(ISettingsRepository<GeneralSettings> generalSettingsRepository)
+        {
+            _generalSettingsRepository = generalSettingsRepository;
+            Theme = GetTheme();
+        }
+
+        public void ApplyTheme()
+        {
+            Theme = GetTheme();
+            ThemeChanged?.Invoke(null, Theme);
+        }
+
+        private ElementTheme GetTheme()
+        {
+            switch (_generalSettingsRepository.SettingsConfig.Theme.ToUpperInvariant())
+            {
+                case "LIGHT":
+                    return ElementTheme.Light;
+                case "DARK":
+                    return ElementTheme.Dark;
+                case "SYSTEM":
+                    return ElementTheme.Default;
+                default:
+                    ManagedCommon.Logger.LogError($"Unexpected theme name: {_generalSettingsRepository.SettingsConfig.Theme}");
+                    return ElementTheme.Default;
+            }
+        }
+    }
+}