Xamarin.Forms-Samples/DataPages Customization/DataPages/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" 
		x:Class="DataPages.App"  
    xmlns:light="clr-namespace:Xamarin.Forms.Themes;assembly=Xamarin.Forms.Theme.Light"
		xmlns:dark="clr-namespace:Xamarin.Forms.Themes;assembly=Xamarin.Forms.Theme.Dark">
    <Application.Resources>
		<!--<ResourceDictionary MergedWith="dark:DarkThemeResources" />-->
        <ResourceDictionary MergedWith="light:LightThemeResources" />
    </Application.Resources>
</Application>