Applies new theme and adds docs link to template (#3235)

* Applies dark theme and adds link to reactjs.org

* Just "learn React"

* Tweaks

packages/react-scripts/template/src/App.css

@@ -4,25 +4,29 @@
 
 .App-logo {
   animation: App-logo-spin infinite 20s linear;
-  height: 80px;
+  height: 40vmin;
 }
 
 .App-header {
-  background-color: #222;
-  height: 150px;
-  padding: 20px;
+  background-color: #282c34;
+  min-height: 100vh;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  font-size: calc(10px + 2vmin);
   color: white;
 }
 
-.App-title {
-  font-size: 1.5em;
-}
-
-.App-intro {
-  font-size: large;
+.App-link {
+  color: #61dafb;
 }
 
 @keyframes App-logo-spin {
-  from { transform: rotate(0deg); }
-  to { transform: rotate(360deg); }
+  from {
+    transform: rotate(0deg);
+  }
+  to {
+    transform: rotate(360deg);
+  }
 }

packages/react-scripts/template/src/App.js

@@ -8,11 +8,18 @@ class App extends Component {
       <div className="App">
         <header className="App-header">
           <img src={logo} className="App-logo" alt="logo" />
-          <h1 className="App-title">Welcome to React</h1>
+          <p>
+            Edit <code>src/App.js</code> and save to reload.
+          </p>
+          <a
+            className="App-link"
+            href="https://reactjs.org"
+            target="_blank"
+            rel="noopener noreferrer"
+          >
+            Learn React
+          </a>
         </header>
-        <p className="App-intro">
-          To get started, edit <code>src/App.js</code> and save to reload.
-        </p>
       </div>
     );
   }

packages/react-scripts/template/src/index.css

 body {
   margin: 0;
   padding: 0;
-  font-family: sans-serif;
+  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen",
+    "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue",
+    sans-serif;
+  -webkit-font-smoothing: antialiased;
+  -moz-osx-font-smoothing: grayscale;
+}
+
+code {
+  font-family: source-code-pro, Menlo, Monaco, Consolas, "Courier New",
+    monospace;
 }