private PaginaPrincipal aparienciaLight(PaginaPrincipal pp)
        {
            pp.Foreground            = night;
            pp.Lista.BorderBrush     = night;
            pp.Lista.Foreground      = night;
            pp.Configura.BorderBrush = night;
            pp.Configura.Foreground  = night;
            pp.Exit.BorderBrush      = night;
            pp.Exit.Foreground       = night;

            string converted  = String.Format("pack://application:,,,/imagenes/logo.png");
            string converted2 = String.Format("pack://application:,,,/imagenes/agenda.png");

            pp.Logo.Source   = new BitmapImage(new Uri(converted, UriKind.Absolute));
            pp.Agenda.Source = new BitmapImage(new Uri(converted2, UriKind.Absolute));

            return(pp);
        }
        // Botón Atrás para navegar a la página PaginaPrincipal

        private void botonAtras(object sender, RoutedEventArgs e)
        {
            System.Windows.Controls.Button b = (System.Windows.Controls.Button)sender;
            if (b != null)
            {
                PaginaPrincipal pp = new PaginaPrincipal(c);

                if (this.Background.ToString().Equals(night.ToString()))
                {
                    pp = aparienciaNight(pp);
                }
                if (this.Background.ToString().Equals(light.ToString()))
                {
                    pp = aparienciaLight(pp);
                }

                this.Content = NavigationService.Navigate(pp);
            }
        }