updated readme of each example with a screenshot/gif and more details about the example.

added more comments to code

using_theme/lib/main.dart

@@ -3,15 +3,27 @@ import 'package:flutter/material.dart';
 void main() {
   runApp(new MaterialApp(
     home: new Scaffold(
+      // AppBar
       appBar: new AppBar(
+        // AppBar Title
         title: new Text("Using Theme"),
       ),
       body: new Container(
           child: new Center(
-            child: new Text("Hello World!", style: new TextStyle(color: Colors.white),),
+            child: new Text(
+              "Hello World!",
+              // Set the text style, text color to white
+              style: new TextStyle(color: Colors.white),
+            ),
           ),
-          decoration: new BoxDecoration(color: Colors.purple)),
+          // Another way to set the background color
+          decoration: new BoxDecoration(color: Colors.lightBlueAccent)),
+    ),
+    // Set the theme's primary color, accent color,
+    theme: new ThemeData(
+      primarySwatch: Colors.blue,
+      accentColor: Colors.lightBlueAccent,
+      backgroundColor: Colors.blueAccent,
     ),
-    theme: new ThemeData(primarySwatch: Colors.deepPurple),
   ));
 }