PHP drupal added css file does not appear in page source

Question

I have enabled LessCSS on my site and I have a node instance where I want to add a specific CSS.

I used the following piece of code:

<?php
    drupal_add_css( '/sites/MyStuff/introduction.css');
?>
<div id="myid" class="box">
    ....some text
</div>

Yet, there is no import or link statement in my page source code for introduction.css. I am not using compression. FireBug does not mention loading an introduction.css file. I have doubled-checked and the file is properly located in the folder.

What could cause this issue? And how to solve it?

P.S.:

The css file contains the following:

.box {
    color: green;
    background-color: red;
}

The styling is not applied on the page.

Answer 1

From https://stackoverflow.com/questions/1410494/drupal-add-css-not-working, the solution is to use CSS Injector. I tried and it worked.

Update

I maintain a page explaining how to use CSS Injector too add CSS to a page.