add tutorial docs in examples

example/lib/custom_theme.dart

@@ -1,12 +1,32 @@
 import 'package:flutter/material.dart';
 import 'package:stream_chat_flutter/stream_chat_flutter.dart';
 
+/// Sixth step of the [tutorial](https://getstream.io/chat/flutter/tutorial/)
+///
+/// The Flutter SDK comes with a fully designed set of widgets which you can customize to fit with your application style and typography.
+/// Changing the theme of Chat widgets works in a very similar way that [MaterialApp] and [Theme] do.
+///
+/// Out of the box all chat widgets use their own default styling, there are two ways to change the styling:
+///
+/// 1. Initialize the [StreamChatTheme] from your existing [MaterialApp] style
+/// 2. Construct a custom theme and provide all the customizations needed
+///
+/// First we create a new Material [Theme] and pick [Colors.green] as swatch color. The theme is then passed to [MaterialApp] as usual.
+///
+/// Then we create a new [StreamChatTheme] from the green theme we just created.
+/// After saving the app you will see the UI will update several widgets to match with the new color.
+///
+/// We also change the message color posted by the current user.
+/// You can perform these more granular style changes using [StreamChatTheme.copyWith].
 void main() async {
-  final client = Client('qk4nn7rpcn75', logLevel: Level.INFO);
+  final client = Client(
+    'b67pax5b2wdq',
+    logLevel: Level.INFO,
+  );
 
   await client.setUser(
-    User(id: 'wild-breeze-7'),
-    'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VyX2lkIjoid2lsZC1icmVlemUtNyJ9.VM2EX1EXOfgqa-bTH_3JzeY0T99ngWzWahSauP3dBMo',
+    User(id: 'falling-mountain-7'),
+    'eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJ1c2VyX2lkIjoiZmFsbGluZy1tb3VudGFpbi03In0.AKgRXHMQQMz6vJAKszXdY8zMFfsAgkoUeZHlI-Szz9E',
   );
 
   runApp(MyApp(client));