CircleButtonStyle.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:local="clr-namespace:WPFMaterialDesign.Controls">



    <local:ColorToSolidColorBrushConverter x:Key="ColorBrushConverter"/>


    <Style TargetType="Button" x:Key="CircleButtonStyle">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="Button" x:Name="CircleButton">
                    <Grid Background="Transparent">
                        <Ellipse Name="Circle" Fill="{TemplateBinding Background}" Width="{TemplateBinding Width}" Height="{TemplateBinding Width}"/>
                        <Viewbox>
                            <Path Stroke="Blue" StrokeThickness="5">
                                <Path.Data>
                                    <LineGeometry StartPoint="0,0" EndPoint="100,0"/>
                                </Path.Data>
                            </Path>
                        </Viewbox>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>