Avalonia/src/Avalonia.Themes.Simple/Controls/PathIcon.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <ControlTheme x:Key="{x:Type PathIcon}"
                TargetType="PathIcon">
    <Setter Property="Foreground" Value="{DynamicResource ThemeForegroundColor}" />
    <Setter Property="Background" Value="Transparent" />
    <Setter Property="Height" Value="{DynamicResource IconElementThemeHeight}" />
    <Setter Property="Width" Value="{DynamicResource IconElementThemeWidth}" />
    <Setter Property="Template">
      <ControlTemplate>
        <Border Background="{TemplateBinding Background}">
          <Viewbox Width="{TemplateBinding Width}"
                   Height="{TemplateBinding Height}">
            <Path Data="{TemplateBinding Data}"
                  Fill="{TemplateBinding Foreground}"
                  Stretch="Uniform" />
          </Viewbox>
        </Border>
      </ControlTemplate>
    </Setter>
  </ControlTheme>
</ResourceDictionary>