(Easy Fix) How to insert div via content.js Google Chrome Extension?

I'm want to create an overlay via content script (content.js) for my google chrome extension. How should I do this?

Here is the code that executes the content script:

chrome.webNavigation.onCommitted.addListener(function() {
chrome.windows.getCurrent(function (currentWindow) {
      chrome.tabs.query({ active: true, windowId: currentWindow.id }, function (activeTabs) {
          activeTabs.map(function (tab) {
              chrome.tabs.executeScript(tab.id, { file: 'contentScript.js', allFrames: false });
          });
      });
  });
}, {url: [{urlMatches : 'https://mail.google.com/'}]});

Here is my contentScript.js

window.addEventListener('DOMContentLoaded', setUpOverlay, false);


function setUpOverlay(){
//Set up overlay div in here
}