Button sample

Test.xml

<Styles xmlns="https://github.com/avaloniaui"
       xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
       xmlns:sys="clr-namespace:System;assembly=mscorlib">


    <Style Selector="Button.Round">
        <Setter Property="Template">
            <ControlTemplate>
                <Border CornerRadius="8" Background="{TemplateBinding Background}">
                    <ContentPresenter Content="{TemplateBinding Content}"/>
                </Border> 
            </ControlTemplate>
        </Setter>        
    </Style>

    <Style Selector="Button.Round:pressed">
        <Setter Property="Template">
            <ControlTemplate>
                <Border Background="Red"
                        CornerRadius="8">
                    <ContentPresenter Content="{TemplateBinding Content}"/>
                </Border>
            </ControlTemplate>
        </Setter>
    </Style>

    <Style Selector="Button.Round:pointerover">
        <Setter Property="Background" Value="Red"/>
    </Style>
</Styles>