How to implement parallax effect in this plugin?

Question

JavaScript

stephband.info/jparallax

{
"manifest_version": 2,
"name": "Режим чтения",
"description": "Для чтения содержимого страницы",
"version": "1.0",

"icons": {
  
  "128": "128.png"

},


"background": {
    "scripts": ["background.js"],
    "persistent": false
  },
 
 "permissions": [ 
 
 	"activeTab",
  "tabs"
 
 ],

      "browser_action": {
        "default_title": "Browser reader",
        "default_icon": "16.png"
    }
    
}

chrome.browserAction.onClicked.addListener( function( tab )  { 
    chrome.tabs.executeScript( null, { file :  "jquery-2.1.1.min.js"  },  function ()  { 
        chrome.tabs.executeScript( null, { file :  "content_script.js"  })
    }) 
});

document.body.style.backgroundColor="#fff";

function parser(tagName) {
  function writeClass() {
    $("p,:header").attr({ class: "readtext" });
  }
  writeClass();
  var res = '';
  $(tagName).find('.readtext').each(function(){
    res += "<p>" + $(this).text() + "</p>";
  });
  return res;
}
var text = parser("body");

$('body').empty().append('<div id="reader" style=" width: 100%; height: auto; background: #fff; padding: 60px; font-size: 15px; padding-top: 30px;">'+text+'</div>');