Need CSS included on a topic page, where to add the code?

[Axeia]

I'm working on a mod to do the highlighting for the [code] tags and to do the actual highlighting I need to add a couple a of CSS classes.
I've added the following code to display.php at the very end (but inside) the display() function.

    //<ServerSideHighlighter>
    $css = file_get_contents(dirname(__FILE__)."/highlighting.css");
    $context['html_headers'] .= '
           <style type="text/css">        
           '.$css.'
           </style>';
    //</ServerSideHighlighter>
This can also be seen at http://pastebin.com/d76e1336e (line 1202-1208)

It however does nothing, on a topic page I can't even find the <style bit which should be there even if the file couldn't be found for the $css variable.
So any suggestions to add it elsewhere, I'm doing something wrong, what's going on?

Yes I know - I should add the stylesheet as <link>, I'll do that later on.

[Matthew K.]

Did you think to try MessageIndex.template.php?

[Axeia]

Was trying to avoid using theme files as to not cause issues when installing new themes etc.

[edit]As the problem seems to be $context['html_headers'] getting either set or being overwritten later on, can I use something like

if(isset($_GET['topic']))
      $context['html_headers'] .= '
           <style type="text/css">        
           '.$css.'
           </style>';

inside Sources/Subs.phps footer_template function?
Does that keep functioning if SEO Friendly urls or Pretty URLs are enabled?

[Arantor]

Why not include that style code inside MessageIndex.php so it can be added when the theme is executed?