[Run] Fix dark mode detection code, plus refactor (#37324)

* Fix risky int cast in dark mode detection.

* Refactored Helper and Manager classes. New unit tests and changes to support Registry access mocking.

* Spelling update.

* Improve documentation for the registry-related classes.

* Fix issue with UpdateTheme raised in review. Enhance documentation. Rewrite tests to use parameterised unit tests, and expand to cover more cases.

src/modules/launcher/PowerLauncher/Services/RegistryService.cs

@@ -0,0 +1,34 @@
+// 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.IO;
+using System.Security;
+using Microsoft.Win32;
+
+namespace PowerLauncher.Services;
+
+#nullable enable
+
+public class RegistryService : IRegistryService
+{
+    /// <inheritdoc/>
+    /// <exception cref="SecurityException">The user does not have the permissions required to read
+    /// from the registry key.</exception>
+    /// <exception cref="IOException">The <see cref="RegistryKey"/> that contains the specified
+    /// value has been marked for deletion.</exception>
+    public object? GetValue(string keyName, string? valueName, object? defaultValue) =>
+        Registry.GetValue(keyName, valueName, defaultValue);
+
+    /// <inheritdoc/>
+    /// <exception cref="SecurityException">The user does not have the permissions required to
+    /// create or modify registry keys.</exception>"
+    public void SetValue(string keyName, string? valueName, object value) =>
+        Registry.SetValue(keyName, valueName, value);
+
+    /// <inheritdoc/>
+    /// <exception cref="SecurityException">The user does not have the permissions required to
+    /// create or modify registry keys.</exception>
+    public void SetValue(string keyName, string? valueName, object value, RegistryValueKind valueKind) =>
+        Registry.SetValue(keyName, valueName, value, valueKind);
+}