round corner and light bg on code areas

c/o @fperez

IPython/frontend/html/notebook/static/css/notebook.css

@@ -198,6 +198,8 b' div.input {'
 div.input_area {
     color: black;
     border: 1px solid #ddd;
+    border-radius: 3px;
+    background: #fafafa;
 }
 
 div.input_prompt {