Windows 8.1 App Change Grid Background

<Page
x:Class="App3.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:App3"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
    <Grid Name="MyGrid">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="6*"/>
            <ColumnDefinition Width="19*"/>
        </Grid.ColumnDefinitions>
        <Button Content="Click Me" HorizontalAlignment="Left" Margin="25.167,80,0,0" VerticalAlignment="Top"
Height="74" Width="142" FontSize="24" FontFamily="Global User Interface"
Click="Button_Click" Grid.Column="1"/>
        <TextBlock x:Name="myTextBlock" HorizontalAlignment="Left" Margin="25.167,246,0,0" TextWrapping="Wrap"
VerticalAlignment="Top" Height="115"  Width="142" FontSize="22" Grid.Column="1"/>
    </Grid>
</Page>

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;
namespace App3 {
    public sealed partial class MainPage: Page {
        public MainPage() {
            this.InitializeComponent();
            this.NavigationCacheMode = NavigationCacheMode.Required;
        }
        protected override void OnNavigatedTo(NavigationEventArgs e) {}
        private void Button_Click(object sender, RoutedEventArgs e) {
            if (MyGrid.Background == null) {
                MyGrid.Background = new SolidColorBrush(Colors.Red);
            } else {
                MyGrid.Background = null;
            }
        }
    }
}