XAML-Map-Control/MapControl/UWP/Themes/Generic.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:map="using:MapControl">

    <Style TargetType="map:MapItemsControl">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate>
                    <ItemsPresenter/>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
        <Setter Property="ItemsPanel">
            <Setter.Value>
                <ItemsPanelTemplate>
                    <map:MapPanel/>
                </ItemsPanelTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style TargetType="ContentControl" x:Key="ContentControlStyle">
        <Setter Property="Foreground" Value="{x:Null}"/>
        <Setter Property="Background" Value="{x:Null}"/>
        <Setter Property="BorderBrush" Value="{x:Null}"/>
        <Setter Property="HorizontalAlignment" Value="Stretch"/>
        <Setter Property="VerticalAlignment" Value="Stretch"/>
        <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
        <Setter Property="VerticalContentAlignment" Value="Stretch"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="ContentControl">
                    <ContentPresenter Content="{TemplateBinding Content}"
                                      ContentTemplate="{TemplateBinding ContentTemplate}"
                                      ContentTemplateSelector="{TemplateBinding ContentTemplateSelector}"
                                      HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                      VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                      Margin="{TemplateBinding Padding}"/>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>

    <Style TargetType="map:MapContentControl" BasedOn="{StaticResource ContentControlStyle}"/>

    <Style TargetType="map:MapItem" BasedOn="{StaticResource ContentControlStyle}"/>

    <Style TargetType="map:Pushpin" BasedOn="{StaticResource ContentControlStyle}">
        <Setter Property="HorizontalAlignment" Value="Left"/>
        <Setter Property="VerticalAlignment" Value="Bottom"/>
        <Setter Property="Padding" Value="5,3"/>
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="map:Pushpin">
                    <Grid>
                        <Grid.RowDefinitions>
                            <RowDefinition/>
                            <RowDefinition Height="Auto"/>
                        </Grid.RowDefinitions>
                        <Border Background="{TemplateBinding Background}"
                                BorderBrush="{TemplateBinding BorderBrush}"
                                BorderThickness="1">
                            <ContentPresenter Content="{TemplateBinding Content}"
                                              ContentTemplate="{TemplateBinding ContentTemplate}"
                                              ContentTemplateSelector="{TemplateBinding ContentTemplateSelector}"
                                              HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                                              VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                                              Margin="{TemplateBinding Padding}"/>
                        </Border>
                        <Path Grid.Row="1"
                              Data="M0.5,-1.2 L0.5,15 10,-1.2"
                              Fill="{TemplateBinding Background}"
                              Stroke="{TemplateBinding BorderBrush}"
                              StrokeThickness="1"/>
                    </Grid>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style>
</ResourceDictionary>