PatriosTheGreat / LinguaLeo paper background

// ==UserScript==
// @name        LinguaLeo paper background
// @namespace   lingualeopaper
// @description Добавляет фон в виде бумаги к контенту в джунглях lingualeo
// @include     http://lingualeo.com/ru/jungle/*
// @version     1
// @grant       none
// ==/UserScript==
$paperBackgroundUrl = "https://2.downloader.disk.yandex.ru/disk/582c04644dd6f9e8ee140a6bce619b2d534dbf2d370172999112906b7d74dd32/inf/fzeCWK2qjwsO4owMChdDK83ztyM3QAlSlMVH_hHO2pRRf3xFt4EFbT9d9ZAFH9vnUUayZl_nS-iwO0MGZ-mCzg%3D%3D?uid=0&filename=blank_older_paper.jpg&disposition=inline&hash=&limit=0&content_type=image%2Fjpeg&tknv=v2";
$contentElement = "#mainContent";

function isEmptyBackground($background){
  return $background == "none" || $background == undefined || $background == '';
}

$(document).on(
  "keypress",
  function(e){
     var keycode = (e.keyCode ? e.keyCode : e.which);
     if(e.shiftKey && keycode == '65'){
        $background = $($contentElement).css("background-image");
        $newBackground = isEmptyBackground($background) ? "url(" + $paperBackgroundUrl + ")" : "none";
        $($contentElement).css("background-image", $newBackground);
     }
  });

// https://yadi.sk/i/1pghXh0_mdw8o