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">

    <ControlTheme TargetType="{x:Type map:MapBase}" x:Key="{x:Type map:MapBase}">
        <Setter Property="Background" Value="White"/>
        <Setter Property="Foreground" Value="Black"/>
    </ControlTheme>

    <ControlTheme TargetType="{x:Type map:Map}" x:Key="{x:Type map:Map}" BasedOn="{StaticResource {x:Type map:MapBase}}"/>

    <ControlTheme TargetType="{x:Type map:MapItemsControl}" x:Key="{x:Type map:MapItemsControl}">
        <Setter Property="Template">
            <ControlTemplate TargetType="{x:Type map:MapItemsControl}">
                <ItemsPresenter ItemsPanel="{TemplateBinding ItemsPanel}"/>
            </ControlTemplate>
        </Setter>
        <Setter Property="ItemsPanel">
            <Setter.Value>
                <ItemsPanelTemplate>
                    <map:MapPanel/>
                </ItemsPanelTemplate>
            </Setter.Value>
        </Setter>
    </ControlTheme>

    <ControlTheme TargetType="{x:Type ContentControl}" x:Key="ContentControlTheme">
        <Setter Property="Background" Value="{Binding Background, RelativeSource={RelativeSource AncestorType=map:MapBase}}"/>
        <Setter Property="BorderBrush" Value="{Binding 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 somehow disables a Template Setter in a Style for a derived type, e.g. MapItem -->
        <!--<Setter Property="Template">
            <ControlTemplate TargetType="{x:Type ContentControl}">
                <ContentPresenter
                    Content="{TemplateBinding Content}"
                    ContentTemplate="{TemplateBinding ContentTemplate}"
                    HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                    VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                    Margin="{TemplateBinding Padding}"/>
            </ControlTemplate>
        </Setter>-->
    </ControlTheme>

    <ControlTheme TargetType="{x:Type map:MapContentControl}" x:Key="{x:Type map:MapContentControl}" BasedOn="{StaticResource ContentControlTheme}">
        <Setter Property="Template">
            <ControlTemplate TargetType="{x:Type map:MapContentControl}">
                <ContentPresenter
                    Content="{TemplateBinding Content}"
                    ContentTemplate="{TemplateBinding ContentTemplate}"
                    HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                    VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                    Margin="{TemplateBinding Padding}"/>
            </ControlTemplate>
        </Setter>
    </ControlTheme>

    <ControlTheme TargetType="{x:Type map:MapItem}" x:Key="{x:Type map:MapItem}" BasedOn="{StaticResource ContentControlTheme}">
        <Setter Property="Template">
            <ControlTemplate TargetType="{x:Type map:MapItem}">
                <ContentPresenter
                    Content="{TemplateBinding Content}"
                    ContentTemplate="{TemplateBinding ContentTemplate}"
                    HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                    VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                    Margin="{TemplateBinding Padding}"/>
            </ControlTemplate>
        </Setter>
    </ControlTheme>

    <ControlTheme TargetType="{x:Type map:Pushpin}" x:Key="{x:Type map:Pushpin}" BasedOn="{StaticResource ContentControlTheme}">
        <Setter Property="HorizontalAlignment" Value="Center"/>
        <Setter Property="VerticalAlignment" Value="Bottom"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="CornerRadius" Value="5"/>
        <Setter Property="Padding" Value="7,5"/>
        <Setter Property="Template">
            <ControlTemplate TargetType="{x:Type 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>