wpftoolkit/ExtendedWPFToolkitSolution_35/Src/WPFToolkit.Extended/Magnifier/Themes/Generic.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:Microsoft.Windows.Controls"
                    xmlns:magConverters="clr-namespace:Microsoft.Windows.Controls.Mag.Converters">

    <!-- =============================================================================== -->
    <!-- Magnifier                                                                       -->
    <!-- =============================================================================== -->

    <magConverters:RadiusConverter x:Key="RadiusConverter"/>
    <magConverters:BorderThicknessToStrokeThicknessConverter x:Key="BorderThicknessToStrokeThicknessConverter"/>

    <Style TargetType="{x:Type local:Magnifier}">
        <Setter Property="IsHitTestVisible" Value="False"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:Magnifier}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="{Binding Path=Radius, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource RadiusConverter}}"/>
                        </Grid.ColumnDefinitions>
                        <Grid.RowDefinitions>
                            <RowDefinition Height="{Binding Path=Radius, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource RadiusConverter}}"/>
                        </Grid.RowDefinitions>
                        <Ellipse Width="{Binding Path=MagnifierWidth, RelativeSource={RelativeSource TemplatedParent}}" Height="{Binding Path=MagnifierHeight, RelativeSource={RelativeSource TemplatedParent}}" Fill="{TemplateBinding Background}"/>
                        <Ellipse Stroke="{Binding Path=BorderBrush, RelativeSource={RelativeSource TemplatedParent}}" StrokeThickness="{Binding Path=BorderThickness, RelativeSource={RelativeSource TemplatedParent}, Converter={StaticResource BorderThicknessToStrokeThicknessConverter}}">
                            <Ellipse.Fill>
                                <VisualBrush Viewbox="{Binding Path=ViewBox, RelativeSource={RelativeSource TemplatedParent}}" 
                                             ViewboxUnits="Absolute"
                                             Visual="{Binding Path=Target, RelativeSource={RelativeSource TemplatedParent}}"/>
                            </Ellipse.Fill>
                        </Ellipse>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

</ResourceDictionary>