XAML-Map-Control/MapControl/Avalonia/Themes/Generic.axaml

<ResourceDictionary
    xmlns="https://github.com/avaloniaui"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:map="clr-namespace:MapControl">

    <ControlTemplate TargetType="ContentControl" x:Key="ContentControlTemplate">
        <ContentPresenter
            Content="{TemplateBinding Content}"
            ContentTemplate="{TemplateBinding ContentTemplate}"
            HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
            VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
            Margin="{TemplateBinding Padding}"/>
    </ControlTemplate>

    <ControlTheme TargetType="ContentControl" x:Key="ContentControlTheme">
        <Setter Property="Background" Value="{CompiledBinding Background, RelativeSource={RelativeSource AncestorType=map:MapBase}}"/>
        <Setter Property="BorderBrush" Value="{CompiledBinding Foreground, RelativeSource={RelativeSource AncestorType=map:MapBase}}"/>
        <Setter Property="HorizontalAlignment" Value="Stretch"/>
        <Setter Property="VerticalAlignment" Value="Stretch"/>
        <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
        <Setter Property="VerticalContentAlignment" Value="Stretch"/>
        <Setter Property="ClipToBounds" Value="False"/>

        <!-- Template Setter here does somehow break a Style for a derived type like MapItem -->
        <!--<Setter Property="Template" Value="{StaticResource ContentControlTemplate}"/>-->
    </ControlTheme>

    <ControlTheme TargetType="map:MapContentControl" x:Key="{x:Type map:MapContentControl}" BasedOn="{StaticResource ContentControlTheme}">
        <Setter Property="Template" Value="{StaticResource ContentControlTemplate}"/>
    </ControlTheme>

    <ControlTheme TargetType="map:MapItem" x:Key="{x:Type map:MapItem}" BasedOn="{StaticResource ContentControlTheme}">
        <Setter Property="Template" Value="{StaticResource ContentControlTemplate}"/>
    </ControlTheme>

    <ControlTheme TargetType="map:Pushpin" x:Key="{x:Type map:Pushpin}" BasedOn="{StaticResource ContentControlTheme}">
        <Setter Property="HorizontalAlignment" Value="Center"/>
        <Setter Property="VerticalAlignment" Value="Bottom"/>
        <Setter Property="HorizontalContentAlignment" Value="Center"/>
        <Setter Property="VerticalContentAlignment" Value="Center"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="CornerRadius" Value="0"/>
        <Setter Property="Padding" Value="6,4"/>
        <Setter Property="Template">
            <ControlTemplate TargetType="map:Pushpin">
                <map:PushpinBorder
                    HorizontalAlignment="{TemplateBinding HorizontalAlignment}"
                    Background="{TemplateBinding Background}"
                    BorderBrush="{TemplateBinding BorderBrush}"
                    BorderWidth="{Binding BorderThickness.Left, RelativeSource={RelativeSource TemplatedParent}}"
                    CornerRadius="{TemplateBinding CornerRadius}"
                    Padding="{TemplateBinding Padding}">
                    <ContentPresenter
                        Content="{TemplateBinding Content}"
                        ContentTemplate="{TemplateBinding ContentTemplate}"
                        HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                        VerticalAlignment="{TemplateBinding VerticalContentAlignment}"/>
                </map:PushpinBorder>
            </ControlTemplate>
        </Setter>
    </ControlTheme>
</ResourceDictionary>