This wiki uses the same extension.
The CSS is embedded in the page header. Look at the HTML code of SyntaxHighlight_GeSHi, you will find:
<style type="text/css">/*<![CDATA[*/
.source-php {line-height: normal;}
.source-php li, .source-php pre {
line-height: normal; border: 0px none white;
}
/**
* GeSHi Dynamically Generated Stylesheet
* --------------------------------------
* Dynamically generated stylesheet for php
* CSS class: source-php, CSS id:
* GeSHi (C) 2004 - 2007 Nigel McNie, 2007 - 2008 Benny Baumann
* (http://qbnz.com/highlighter/ and http://geshi.org/)
* --------------------------------------
*/
.php.source-php .de1, .php.source-php .de2 {font: normal normal 1em/1.2em monospace; margin:0; padding:0; background:none; vertical-align:top;}
.php.source-php {font-family:monospace;}
.php.source-php .imp {font-weight: bold; color: red;}
// ...
And now look here:
http://www.mediawiki.org/w/index.php?title=Extension%3ASyntaxHighlight_GeSHi&action=historysubmit&diff=448677&oldid=446097
This is the page I'm referring to.
Below the diff you find the "Latest revision as of 15:49, 28 October 2011", which is statically integrated into the page.
On my wiki, this part of the page is loaded dynamically using AJAX. But it fails to copy the loaded page's embedded CSS to the overall page and therefore the sourcecode looks very ugly (not just not highlighted, but when you use line numbers, every single line has a border around it).
So, somehow the AJAX part must have been disabled here.