theme.xaml

<controls:XamlLineArt LineArt="{TemplateBinding LineArt}" HorizontalAlignment="Center"
                    MaxWidth="{TemplateBinding ImageMaxWidth}" MaxHeight="{TemplateBinding ImageMaxHeight}"
                    Margin="{TemplateBinding ImageMargin}" DockPanel.Dock="{TemplateBinding ImagePosition}">
    <controls:XamlLineArt.Triggers>
        <DataTrigger Value="true" Binding="{Binding IsSelected, RelativeSource={RelativeSource AncestorType={x:Type controls:LineArtButton}}}">
            <!-- This seems to set Stroke on LineArtButton and not XamlLineArt... :( -->
            <Setter Property="Stroke" Value="Blue" />
        </DataTrigger>
    </controls:XamlLineArt.Triggers>
</controls:XamlLineArt>