rename App.css to App.scss and remove template gen styles

Author: amliu

src/App.css

@@ -1,6 +1,7 @@
 import React from 'react';
 import logo from './logo.svg';
 import './App.css';
+import './App.scss';
 
 function App() {
   return (

src/App.js

@@ -0,0 +1,7 @@
+.App {
+  width: 100%;
+  max-width: 540px;
+  padding: 32px;
+
+  margin: 0 auto;
+}