Object reference not set to an instance of an object when using css styling with xamarin forms

Question

I tried to set style to my page using css but i got this exception

Object reference not set to an instance of an object

MainPage.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="XRetailMobile.Views.MainPage" 
             xmlns:local="clr-namespace:XRetailMobile.Views.ContentViews"
             NavigationPage.HasNavigationBar="False">
    <ContentPage.Resources>
        <StyleSheet Source="/Styles/style.css"/>
    </ContentPage.Resources>
    <ContentPage.Content>
        <Grid HorizontalOptions="Fill" VerticalOptions="Fill">           
            <local:ProductBackToolbar StyleClass="toolbarStyle" VerticalOptions="Start">
            </local:ProductBackToolbar>

        </Grid>
    </ContentPage.Content>

</ContentPage>

ProductBackToolbar.xaml(contentview)

<ContentView.Content>
    <StackLayout HorizontalOptions="Fill" Orientation="Horizontal">
        <Image Source="back" HeightRequest="35" VerticalOptions="Center" WidthRequest="35" HorizontalOptions="StartAndExpand"></Image>
        <Image Source="share" HeightRequest="35" VerticalOptions="Center" WidthRequest="35" HorizontalOptions="End"></Image>
        <Image Source="shoppingbag" HeightRequest="35" WidthRequest="35" VerticalOptions="Center" HorizontalOptions="End"></Image>
    </StackLayout>
</ContentView.Content>

style.css Located in Style Folder and its build action is EmbeddedResources

^contentpage {
    background-color: greenyellow;
}
.toolbarStyle {
    background-color: blue;
    margin: 20px;
    padding:20px;
}

if i comment margin and padding it's work fine what's the wrong ?