Application resources

Hi guys,
I have a question about Application Resources. I should change the style of my application. I tried to follow the instruction on https://developer.xamarin.com/guides/xamarin-forms/xaml/resource-dictionaries/ but nothing changed in my application.

I removed App.cs and I created a new ContentPage and I changed it like that:

App.xaml

<?xml version="1.0" encoding="UTF-8"?>
<Application xmlns="http://xamarin.com/schemas/2014/forms" 
     xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
     xmlns:theme="clr-namespace:Xamarin.Forms.Themes;assembly=Xamarin.Forms.Theme.Light" 
     x:Class="myInventories.App">
    <Application.Resources>
    <ResourceDictionary>
        <Color x:Key="PageBackgroundColor">Yellow</Color>
        <Color x:Key="HeadingTextColor">Black</Color>
        <Color x:Key="NormalTextColor">Blue</Color>
        <Style x:Key="LabelPageHeadingStyle" TargetType="Label">
            <Setter Property="FontAttributes" Value="Bold" />
            <Setter Property="HorizontalOptions" Value="Center" />
            <Setter Property="TextColor" Value="{StaticResource HeadingTextColor}" />
        </Style>
        <Style TargetType="Entry">
            <Setter Property="HorizontalOptions" Value="Fill" />
            <Setter Property="VerticalOptions" Value="CenterAndExpand" />
            <Setter Property="BackgroundColor" Value="Yellow" />
            <Setter Property="FontAttributes" Value="Italic" />
            <Setter Property="TextColor" Value="Blue" />
        </Style>
    </ResourceDictionary>
    </Application.Resources>
</Application>

App.xaml.cs

public partial class App : Application
{
    public App()
    {
        MainPage = new StartPage();
    }

    public static Page GetMainPage()
    {
        return new RootPage();
    }

    protected override void OnStart()
    {
        // Handle when your app starts
    }

    protected override void OnSleep()
    {
        // Handle when your app sleeps
    }

    protected override void OnResume()
    {
        // Handle when your app resumes
    }
}

I was expecting the background application Yellow. What is wrong? Thank you in advance.