Can I inject a CSS file programmatically using a content script js file?

Can I inject a CSS file programmatically using a content script js file?

It is possible for me to inject the css when the js file is linked to my popup.html. The problem is I have to click on the button to open the popup to inject the css. I want it to happen automatically and in the background.

What happens in my code...

1. Get a variable from a MySQL database through a XMLHttpRequest
2. Call the function, "processdata()"
3. "processdata" Will process the data from the XMLHttpRequest. More specifically, split the variable, put it into 2 different variables and make them global
4. I call the function, "click()"
5. "click" Then will set the css after 1250 milliseconds using setTimeout
6. I use chrome.tabs.insertCSS to insert the css. The css name is the variable, "currenttheme"

As I mentioned before it does work using the popup. But the popup has to be opened before the CSS gets injected.

How do I make this all happen automatically, without any user interaction?

Here is my code:

    function getData() {
if (window.XMLHttpRequest)
      {// code for IE7+, Firefox, Chrome, Opera, Safari
      xmlhttp=new XMLHttpRequest();
      }
    else
      {// code for IE6, IE5
      xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
      }
    xmlhttp.onreadystatechange=function()
      {
      if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {
        user_data = xmlhttp.responseText;
        window.user_data = user_data;
        processdata();
        }
      }
    xmlhttp.open("GET","http://localhost:8888/g_dta.php",true);
    xmlhttp.send();
}

getData();

function processdata() {
  var downdata = user_data.split('|||||');
  var installedthemes = downdata[0];
  var currenttheme = downdata[1].toString();
  window.currenttheme = currenttheme;
  click();
  }

function click(e) { 
      function setCSS() {
          chrome.