Merging Resources at runtime. Docs Guide: https://developer.xamarin.com/guides/xamarin-forms/xaml/resource-dictionaries/#Merged_Resource_Dictionaries

MyXamlStyles.xaml

<?xml version="1.0" encoding="utf-8" ?>
<ResourceDictionary xmlns="http://xamarin.com/schemas/2014/forms"
                    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                    x:Class="My.Project.Styles">
    <Style TargetType="Label">
        <Setter Property="TextColor" Value="Blue" />
    </Style>
  ...
</ResourceDictionary>

page.xaml.cs

using My.Project.Styles;
...

if (App.Current.Resources == null)
  App.Current.Resources = new ResourceDictionary();
App.Current.Resources.MergedWith = typeof(MyXamlStyles);

page2.xaml

<Application ... xmlns:myTheme="clr-namespace:My.Project.Styles">
  <Application.Resources>
    <ResourceDictionary MergedWith="myTheme:MyXamlStyles" />
  </Application.Resources>
  ...
</Application