ButtonCornerRadius.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="Xamarin.Forms.Xaml.UnitTests.ButtonCornerRadius">
    <ContentPage.Resources>
        <Style TargetType="Button">
            <Setter Property="TextColor" Value="White"></Setter>
        </Style>
        <Style x:Key="ButtonStyle" TargetType="Button">
            <Setter Property="CornerRadius" Value="0"></Setter>
            <Setter Property="TextColor" Value="Yellow"></Setter>
        </Style>
    </ContentPage.Resources>
    <ContentPage.Content>
        <Button x:Name="Button0" Style="{StaticResource ButtonStyle}" />
    </ContentPage.Content>
</ContentPage>