pokemon_meme_for_inbox_zero.user.js

// ==UserScript==
// @name         Pokemon meme for inbox zero
// @namespace    http://example.com
// @author       You
// @match        https://mail.google.com/*
// @version      1.1
// @grant        GM_addStyle
// ==/UserScript==

GM_addStyle('tr.TD td.my_congratulations { font: 25.6px Serif; padding: 8px 0 36px 0; }');

function congratulations() {
  var node = document.querySelector('tr.TD td.TC');
  if (node && node.innerText == 'No new mail!') {
    node.innerHTML = '' +
        '<p>So excited to see this happening!</p>' +
        '<img src="http://media1.giphy.com/media/MhHXeM4SpKrpC/giphy.gif">';
    node.setAttribute('class', node.getAttribute('class') + ' my_congratulations');
  }
}

var token = null;
document.addEventListener(
  "DOMNodeInserted",
  function() { clearTimeout(token); token = setTimeout(congratulations, 1); },
  false
);