Add chapter 10 sample project

Chapter10/TemplateStudioSampleApp/Services/ThemeSelectorService.cs

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+using Microsoft.UI.Xaml;
+
+using TemplateStudioSampleApp.Contracts.Services;
+using TemplateStudioSampleApp.Helpers;
+
+namespace TemplateStudioSampleApp.Services;
+
+public class ThemeSelectorService : IThemeSelectorService
+{
+    private const string SettingsKey = "AppBackgroundRequestedTheme";
+
+    public ElementTheme Theme { get; set; } = ElementTheme.Default;
+
+    private readonly ILocalSettingsService _localSettingsService;
+
+    public ThemeSelectorService(ILocalSettingsService localSettingsService)
+    {
+        _localSettingsService = localSettingsService;
+    }
+
+    public async Task InitializeAsync()
+    {
+        Theme = await LoadThemeFromSettingsAsync();
+        await Task.CompletedTask;
+    }
+
+    public async Task SetThemeAsync(ElementTheme theme)
+    {
+        Theme = theme;
+
+        await SetRequestedThemeAsync();
+        await SaveThemeInSettingsAsync(Theme);
+    }
+
+    public async Task SetRequestedThemeAsync()
+    {
+        if (App.MainWindow.Content is FrameworkElement rootElement)
+        {
+            rootElement.RequestedTheme = Theme;
+
+            TitleBarHelper.UpdateTitleBar(Theme);
+        }
+
+        await Task.CompletedTask;
+    }
+
+    private async Task<ElementTheme> LoadThemeFromSettingsAsync()
+    {
+        var themeName = await _localSettingsService.ReadSettingAsync<string>(SettingsKey);
+
+        if (Enum.TryParse(themeName, out ElementTheme cacheTheme))
+        {
+            return cacheTheme;
+        }
+
+        return ElementTheme.Default;
+    }
+
+    private async Task SaveThemeInSettingsAsync(ElementTheme theme)
+    {
+        await _localSettingsService.SaveSettingAsync(SettingsKey, theme.ToString());
+    }
+}