How to apply a theme in Xamarin.Forms

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="ThemeDemo.App">
	<Application.Resources>
        <ResourceDictionary>
            <Color x:Key="BackgroundColorDark">#121212</Color>
            <Color x:Key="BackgroundColorLight">#ffffff</Color>
            <Color x:Key="BackgroundColorBrown">#292220</Color>
            
            <Color x:Key="BarBackgroundColorDark">#2e2e2e</Color>
            <Color x:Key="BarBackgroundColorLight">#d3d3d3</Color>
            <Color x:Key="BarBackgroundColorBrown">#3c3835</Color>
            
            <Color x:Key="TextColorDark">#ffffff</Color>
            <Color x:Key="TextColorLight">#444444</Color>
            <Color x:Key="TextColorBrown">#ffffff</Color>
        </ResourceDictionary>
    </Application.Resources>
</Application>

App.xaml.cs

using Xamarin.Forms;

namespace ThemeDemo
{
    public enum Theme
    {
        Dark, Light, Brown
    }

    public partial class App : Application
    {
        static readonly string[] resources = new string[] { "BackgroundColor", "BarBackgroundColor", "TextColor" };

        Theme theme;
        public Theme Theme
        {
            get => theme;
            set
            {
                theme = value;

                foreach (var resource in resources)
                {
                    Resources[resource] = Resources[$"{resource}{theme}"];
                }
            }
        }

        public App()
        {
            InitializeComponent();

            Theme = Theme.Dark;

            var page = new NavigationPage(new MainPage());
            
            // in C# code
            page.SetDynamicResource(NavigationPage.BarTextColorProperty, "TextColor");
            page.SetDynamicResource(NavigationPage.BarBackgroundColorProperty, "BarBackgroundColor");

            MainPage = page;
        }
    }
}

MainPage.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="ThemeDemo.MainPage"
             Title="Theme Demo">
    <ContentPage.Content>
        <StackLayout BackgroundColor="{DynamicResource BackgroundColor}">
            <Button Text="Change theme!"
                    FontSize="Large"
                    Clicked="Button_Clicked"
                    VerticalOptions="CenterAndExpand"
                    HorizontalOptions="CenterAndExpand" />
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

MainPage.xaml.cs

using System;
using Xamarin.Forms;

namespace ThemeDemo
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
        }

        int index = 1;
        private void Button_Clicked(object sender, EventArgs e)
        {
            var app = App.Current as App;

            app.Theme = (Theme)(index++ % 3);
        }
    }
}