Added style

popup.html

       body {
         font-family: "Segoe UI", "Lucida Grande", Tahoma, sans-serif;
         font-size: 100%;
-        /* avoid an excessively wide status text */
-        white-space: pre;
         text-overflow: ellipsis;
         overflow: hidden;
-        max-width: 400px;
+        width: 400px;
+        text-align:center;
+      }
+      button {
+        width: 100%;
+        height: 50px;
+        margin-top: 10px;
       }
     </style>
     <script src="jquery.js"></script>