Border on a TextBox

Take a look at this XAML:
    <Grid x:Name="LayoutRoot" Background="White">
       
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="10"></ColumnDefinition>
            <ColumnDefinition Width="*"></ColumnDefinition>
            <ColumnDefinition Width="10"></ColumnDefinition>
        </Grid.ColumnDefinitions>
       
        <Grid.RowDefinitions>
            <RowDefinition Height="10"&g ...