<UserControl.Resources>
        <Style TargetType="Button">
            <Setter Property="Width" Value="18" />
            <Setter Property="Height" Value="18" />
            <Setter Property="Padding" Value="0" />
            <Setter Property="Margin" Value="0,0,3,0" />
            <!-- Remove default button border. -->
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="Button">
                        <Border Background="Transparent"
                                BorderThickness="1"
                                BorderBrush="{Binding Foreground, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=ContentControl}}"
                                Padding="2">
                            <ContentPresenter/>
                        </Border>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </UserControl.Resources>