ThemePickerViewModel

ThemePickerViewModel.cs

public class ThemePickerViewModel
{
  public IList<AppThemeViewModel> AppThemes { get; set; } = new List<AppThemeViewModel> {
    new AppThemeViewModel() { Key = (int)OSAppTheme.Unspecified },
    new AppThemeViewModel() { Key = (int)OSAppTheme.Light },
    new AppThemeViewModel() { Key = (int)OSAppTheme.Dark }
  };

  public ICommand SetAppThemeCommand { get; set; }

  public ThemePickerViewModel()
  {
      SetAppThemeCommand = new Command<AppThemeViewModel>((x) => SetAppTheme(x));
  }

  void SetAppTheme(AppThemeViewModel theme)
  {
      Application.Current.UserAppTheme = (OSAppTheme)theme.Key;
      Preferences.Set(Settings.AppTheme, theme.Key);
  }
}