<Window xmlns="https://github.com/avaloniaui" MinWidth="500" MinHeight="300">
    <Window.Styles>
        <Style Selector="Button.circle">
            <Setter Property="Button.Template">
                <Setter.Value>
                        <ControlTemplate>
                            <Grid>
                                <Grid.Styles>
                                </Grid.Styles>
                                <Ellipse Name="Ellipse" Stroke="Black" StrokeThickness="2">

                                </Ellipse>
                                <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"  Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}"/>
                            </Grid>
                        </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
        <Style Selector="Button.circle /template/ Ellipse">
            <Setter Property="Fill" Value="Red"/>
        </Style>
        <Style Selector="Button.circle:pointerover /template/ Ellipse">
            <Setter Property="Fill" Value="Green"/>
        </Style>
        <Style Selector="Button.circle:pressed  /template/ Ellipse">
            <Setter Property="Fill" Value="Blue"/>
        </Style>
        <Style Selector="Button.circle:pressed  /template/ ContentPresenter">
            <Setter Property="Background" Value="Blue"/>
        </Style>
    </Window.Styles>
        <Button Width="100" Height="100" Classes="circle">I'm a button</Button>
</Window>