Style for FocuVisualStyle not applied

In my application I want that a CheckBox if it get's the Keyboardfocus should not have a dotted border. Therfor I have set the FocusVisualStyle to null. The definition of the CheckBox looks like:

<CheckBox Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="2" Margin="0,5,0,10" Style="{StaticResource MyCheckBoxStyle}"
          IsChecked="{Binding UseOriginalPath, UpdateSourceTrigger=PropertyChanged}" FocusVisualStyle="{x:Null}" >
    <TextBlock TextWrapping="Wrap" Text="Use the original path"/>
</CheckBox>

My custom style for the CheckBox looks like:

<Style TargetType="CheckBox" x:Key="MyCheckBoxStyle">
    <Setter Property="ContentTemplate">
        <Setter.Value>
            <DataTemplate>
                <Grid Margin="2.5">
                    <Border BorderThickness="0" BorderBrush="Green" Margin="-25,-2.5,-6,-2.5" x:Name="border"/>
                    <ContentPresenter Content="{TemplateBinding Content}"/>
                </Grid>
                <DataTemplate.Triggers>
                    <DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=CheckBox}, Path=IsFocused}" Value="True">
                        <Setter TargetName="border" Property="BorderThickness" Value="1.5"/>
                    </DataTrigger>
                </DataTemplate.Triggers>
            </DataTemplate>
        </Setter.Value>
    </Setter>
</Style>

Until here everything just works as expected. If I focus the CheckBox with the Tab-Key there is no dotted border around it.

But if I remove the FocusVisualStyle="{x:Null}" from the CheckBox definition and want to move it to the syle it wont work.

So the definition of the CheckBox now looks like:

<CheckBox Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="2" Margin="0,5,0,10" Style="{StaticResource MyCheckBoxStyle}"
          IsChecked="{Binding UseOriginalPath, UpdateSourceTrigger=PropertyChanged}">
    <TextBlock TextWrapping="Wrap" Text="Use the original path"/>
</CheckBox>

And the Style looks like:

<Style TargetType="CheckBox" x:Key="MyCheckBoxStyle">
    <Setter Property="ContentTemplate">
        <Setter.Value>
            <DataTemplate>
                <Grid Margin="2.5">
                    <Border BorderThickness="0" BorderBrush="#79B100" Margin="-25,-2.5,-6,-2.5" x:Name="border"/>
                    <ContentPresenter Content="{TemplateBinding Content}"/>
                </Grid>
                <DataTemplate.Triggers>
                    <DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType=CheckBox}, Path=IsFocused}" Value="True">
                        <Setter TargetName="border" Property="BorderThickness" Value="1.5"/>
                    </DataTrigger>
                </DataTemplate.Triggers>
            </DataTemplate>
        </Setter.Value>
    </Setter>
    <Setter Property="FocusVisualStyle" Value="{x:Null}"/>
</Style>

Now if I focus the CheckBox with the Tab-Key there are the dottet borders. I don't understand what I'm doing wrong here?