AppShell.xaml

<Style x:Key="FlyoutHeaderStyle" TargetType="Label">
    <Setter Property="FontSize" Value="Default" />
    <Setter Property="Margin" Value="15,0" />
    <Setter Property="TextColor" Value="Black" />
    <Setter Property="FontAttributes" Value="Bold" />
    <Setter Property="HorizontalOptions" Value="Start" />
</Style>

<Style x:Key="FlyoutItemStyle" TargetType="Label">
    <Setter Property="FontSize" Value="Default" />
    <Setter Property="Margin" Value="15,1" />
    <Setter Property="TextColor" Value="Red" />
    <Setter Property="HorizontalOptions" Value="Start" />
</Style>

<Style x:Key="FlyoutMenuStyle" TargetType="Label">
    <Setter Property="FontSize" Value="Default" />
    <Setter Property="TextColor" Value="Gray" />
    <Setter Property="FontAttributes" Value="Italic" />
    <Setter Property="Margin" Value="15,0" />
    <Setter Property="HorizontalOptions" Value="Start" />
</Style>

<DataTemplate x:Key="FlyoutItemTemplate">
    <StackLayout Orientation="Horizontal">
        <BoxView BackgroundColor="{Binding FlyoutColor}" Margin="5,0,5,0" WidthRequest="4" VerticalOptions="CenterAndExpand" />
        <Label 
            FontFamily="{StaticResource MaterialFontFamily}" 
            Text="{StaticResource {Binding ItemReferenceImage}}" 
            Margin="0,0,10,0"/>
        <Label
            Style="{StaticResource FlyoutItemStyle}"
            Text="{Binding Title}"
            TextColor="Black"
            VerticalOptions="Center" />
    </StackLayout>
</DataTemplate>
<DataTemplate x:Key="FlyoutHeaderTemplate">
    <StackLayout Orientation="Horizontal" Margin="5,0,5,0">
        <BoxView BackgroundColor="{Binding FlyoutColor}" WidthRequest="4" VerticalOptions="CenterAndExpand" />
        <StackLayout Orientation="Vertical">
            <Label
                Style="{StaticResource FlyoutHeaderStyle}"
                Text="{Binding Title}"
                TextColor="Black"
                VerticalOptions="Center" />
            <BoxView
                Margin="15,2,25,0"
                BackgroundColor="{StaticResource HackneyRed}"
                HeightRequest="1"
                HorizontalOptions="FillAndExpand"/>
        </StackLayout>
    </StackLayout>

</DataTemplate>
<controls:NavigationFlyoutItemTemplateSelector
    x:Key="FlyoutTemplateSelector"
    NavigationHeaderTemplate="{StaticResource FlyoutHeaderTemplate}"
    NavigationItemTemplate="{StaticResource FlyoutItemTemplate}" />