WPF布局缓冲颜色

<Grid Name="grid1">

            <Grid.RowDefinitions >

                <RowDefinition Height="*"></RowDefinition>

                <RowDefinition Height="auto"></RowDefinition>

                <RowDefinition Height="*"></RowDefinition>

            </Grid.RowDefinitions>

            <TextBox Grid.Row="0" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Margin="10,10,13,10"

                Name="txtQuestion"  TextWrapping="Wrap" FontFamily="Verdana" FontSize="24">

                [pleass using english]

            </TextBox>

            <Button Grid.Row="1" VerticalAlignment="Top" HorizontalAlignment="Left" Margin="10,0,0,20" Width="127" Height="34"

                    Name="btn" Click="btn_Click">

            </Button>

            <TextBox Grid.Row="2" VerticalAlignment="Stretch" HorizontalAlignment="Stretch" Margin="10,10,13,10"

                Name="txtAnw"  TextWrapping="Wrap" FontFamily="Verdana" FontSize="24" Foreground="Red">

                [pleass using english]

            </TextBox>

            <Grid.Background>

                <LinearGradientBrush>

                    <LinearGradientBrush.GradientStops>

                        <GradientStop Offset="0.00" Color="Blue"></GradientStop>

                        <GradientStop Offset="0.50" Color="Red"></GradientStop>

                        <GradientStop Offset="1.00" Color="Black"></GradientStop>

                    </LinearGradientBrush.GradientStops>

                </LinearGradientBrush>

            </Grid.Background>

        </Grid>