How can I round off the corners of a CheckBox template in WPF Blend?

So I created a CheckBox template in Visual Studio Blend and then wanted to round off the corners. However, there is only one border in the whole template that only rounds off the checkbox itself: Picture but not the Background, i made the Background Colorized because I want to turn this CheckBox into a slide / toggle button. it because it is a grid.

Template:

<Window.Resources>
    <Style x:Key="FocusVisual">
        <Setter Property="Control.Template">
            <Setter.Value>
                <ControlTemplate>
                    <Rectangle Margin="2" SnapsToDevicePixels="true" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    <SolidColorBrush x:Key="OptionMark.Static.Background" Color="#FFFFFFFF"/>
    <SolidColorBrush x:Key="OptionMark.Static.Border" Color="#FF707070"/>
    <Style x:Key="OptionMarkFocusVisual">
        <Setter Property="Control.Template">
            <Setter.Value>
                <ControlTemplate>
                    <Rectangle Margin="14,0,0,0" SnapsToDevicePixels="true" Stroke="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}" StrokeThickness="1" StrokeDashArray="1 2"/>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
    <SolidColorBrush x:Key="OptionMark.MouseOver.Background" Color="#FFF3F9FF"/>
    <SolidColorBrush x:Key="OptionMark.MouseOver.Border" Color="#FF5593FF"/>
    <SolidColorBrush x:Key="OptionMark.MouseOver.Glyph" Color="#FF212121"/>
    <SolidColorBrush x:Key="OptionMark.Disabled.Background" Color="#FFE6E6E6"/>
    <SolidColorBrush x:Key="OptionMark.Disabled.Border" Color="#FFBCBCBC"/>
    <SolidColorBrush x:Key="OptionMark.Disabled.Glyph" Color="#FF707070"/>
    <SolidColorBrush x:Key="OptionMark.Pressed.Background" Color="#FFD9ECFF"/>
    <SolidColorBrush x:Key="OptionMark.Pressed.Border" Color="#FF3C77DD"/>
    <SolidColorBrush x:Key="OptionMark.Pressed.Glyph" Color="#FF212121"/>
    <SolidColorBrush x:Key="OptionMark.Static.Glyph" Color="#FF212121"/>
    <Style x:Key="CheckBoxStyle1" TargetType="{x:Type CheckBox}">
        <Setter Property="FocusVisualStyle" Value="{StaticResource FocusVisual}"/>
        <Setter Property="Background" Value="{StaticResource OptionMark.Static.Background}"/>
        <Setter Property="BorderBrush" Value="{StaticResource OptionMark.Static.Border}"/>
        <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type CheckBox}">
                    <Grid x:Name="templateRoot" Background="{x:Null}" SnapsToDevicePixels="True" Margin="-0.729,-0.854,0.75,0">
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto"/>
                            <ColumnDefinition Width="*"/>
                        </Grid.ColumnDefinitions>
                        <Border x:Name="checkBoxBorder" CornerRadius="25" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="0" Background="Red" HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}" Margin="0.931,0.912,0,-0.092" VerticalAlignment="{TemplateBinding VerticalContentAlignment}" Height="52.784" Width="56.319" Grid.ColumnSpan="2">
                            <Grid x:Name="markGrid">
                                <Path x:Name="optionMark" Data="F1 M 9.97498,1.22334L 4.6983,9.09834L 4.52164,9.09834L 0,5.19331L 1.27664,3.52165L 4.255,6.08833L 8.33331,1.52588e-005L 9.97498,1.22334 Z " Fill="{StaticResource OptionMark.Static.Glyph}" Margin="1" Opacity="0" Stretch="None"/>
                                <Rectangle x:Name="indeterminateMark" Fill="#FF0C0C0C" Margin="2" Opacity="0" Stroke="{x:Null}"/>
                            </Grid>
                        </Border>
                    </Grid>
                    <ControlTemplate.Triggers>
                        <Trigger Property="HasContent" Value="true">
                            <Setter Property="FocusVisualStyle" Value="{StaticResource OptionMarkFocusVisual}"/>
                            <Setter Property="Padding" Value="4,-1,0,0"/>
                        </Trigger>
                        <Trigger Property="IsMouseOver" Value="true">
                            <Setter Property="Background" TargetName="checkBoxBorder" Value="{StaticResource OptionMark.MouseOver.Background}"/>
                            <Setter Property="BorderBrush" TargetName="checkBoxBorder" Value="{StaticResource OptionMark.MouseOver.Border}"/>
                            <Setter Property="Fill" TargetName="optionMark" Value="{StaticResource OptionMark.MouseOver.Glyph}"/>
                            <Setter Property="Fill" TargetName="indeterminateMark" Value="{StaticResource OptionMark.MouseOver.Glyph}"/>
                        </Trigger>
                        <Trigger Property="IsEnabled" Value="false">
                            <Setter Property="Background" TargetName="checkBoxBorder" Value="{StaticResource OptionMark.Disabled.Background}"/>
                            <Setter Property="BorderBrush" TargetName="checkBoxBorder" Value="{StaticResource OptionMark.Disabled.Border}"/>
                            <Setter Property="Fill" TargetName="optionMark" Value="{StaticResource OptionMark.Disabled.Glyph}"/>
                            <Setter Property="Fill" TargetName="indeterminateMark" Value="{StaticResource OptionMark.Disabled.Glyph}"/>
                        </Trigger>
                        <Trigger Property="IsPressed" Value="true">
                            <Setter Property="Background" TargetName="checkBoxBorder" Value="{StaticResource OptionMark.Pressed.Background}"/>
                            <Setter Property="BorderBrush" TargetName="checkBoxBorder" Value="{StaticResource OptionMark.Pressed.Border}"/>
                            <Setter Property="Fill" TargetName="optionMark" Value="{StaticResource OptionMark.Pressed.Glyph}"/>
                            <Setter Property="Fill" TargetName="indeterminateMark" Value="{StaticResource OptionMark.Pressed.Glyph}"/>
                        </Trigger>
                        <Trigger Property="IsChecked" Value="true">
                            <Setter Property="Opacity" TargetName="optionMark" Value="1"/>
                            <Setter Property="Opacity" TargetName="indeterminateMark" Value="0"/>
                        </Trigger>
                        <Trigger Property="IsChecked" Value="{x:Null}">
                            <Setter Property="Opacity" TargetName="optionMark" Value="0"/>
                            <Setter Property="Opacity" TargetName="indeterminateMark" Value="1"/>
                        </Trigger>
                    </ControlTemplate.Triggers>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</Window.Resources>
<Grid>
    <CheckBox Content="CheckBox" HorizontalAlignment="Left" Height="53" Margin="219,155,0,0" VerticalAlignment="Top" Width="185" Style="{DynamicResource CheckBoxStyle1}"/>
</Grid>

You can create another Border around Grid:

<ControlTemplate TargetType="{x:Type CheckBox}">
    <Border BorderBrush="{TemplateBinding BorderBrush}" CornerRadius="10" BorderThickness="{TemplateBinding BorderThickness}">
        <Grid x:Name="templateRoot" Background="{x:Null}" SnapsToDevicePixels="True" Margin="-0.729,-0.854,0.75,0">
        ....
        </Grid>
    </Border>
    <ControlTemplate.Triggers>
    ...
    </ControlTemplate.Triggers>
</ControlTemplate>