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

<Styles xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
  <Style Selector="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 Height="{TemplateBinding Height}"
                   Width="{TemplateBinding Width}">
            <Path Fill="{TemplateBinding Foreground}"
                  Data="{TemplateBinding Data}"
                  Stretch="Uniform" />
          </Viewbox>
        </Border>
      </ControlTemplate>
    </Setter>
  </Style>
</Styles>