Example of Global Styles in Xamarin.Forms applications - http://julianocustodio.com/global-styles

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="DemoTimeLine.App">
     <Application.Resources>
        <ResourceDictionary>

            <!--Colors-->
            <Color x:Key="ColorText">#3030E8</Color>
            <Color x:Key="ColorButton">#20E82A</Color>


            <!--Font Families-->
            <OnPlatform x:TypeArguments="x:String" x:Key="RegularFontFamily">
                <On Platform="Android">sans-serif</On>
                <On Platform="iOS">HelveticaNeue</On>
            </OnPlatform>
            <OnPlatform x:TypeArguments="x:String" x:Key="MediumFontFamily">
                <On Platform="Android">sans-serif-medium</On>
                <On Platform="iOS">HelveticaNeue-Medium</On>
            </OnPlatform>


            <!--Styles-->
            <Style x:Key="TitleStyle" TargetType="Label">
                <Setter Property="TextColor" Value="{StaticResource ColorText}" />
                <Setter Property="Font" Value="{StaticResource MediumFontFamily}" />
            </Style>

            <Style x:Key="ButtonStyle" TargetType="Button">
                <Setter Property="TextColor" Value="{StaticResource ColorText}" />
                <Setter Property="Font" Value="{StaticResource RegularFontFamily}" />
                <Setter Property="BackgroundColor" Value="{StaticResource ColorButton}"/>
            </Style>

        </ResourceDictionary>
    </Application.Resources>
</Application>