avaloniaa/AvaloniaApplication3/Styles1.axaml

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Design.PreviewWith>
    <Border Padding="20">
      <!-- Add Controls for Previewer Here -->
    </Border>
  </Design.PreviewWith>

	<Style Selector="Button">

		<Setter Property="Background" Value="LightSteelBlue"/>
		<Setter Property="CornerRadius" Value="15"/>
		<Setter Property="BorderBrush" Value="DarkGray"/>
		<Setter Property="BorderThickness" Value="2"/>
		<Setter Property="Padding" Value="10"/>
		<Setter Property="Width" Value="300"/>
		<Setter Property="HorizontalContentAlignment" Value="Center"/>
		<!-- Add Styles Here -->
		<Setter Property="Foreground" Value="White"/>
	
	</Style>
	
</Styles>