First, you need to create an advanced custom theme by following the instructions here:
http://www.question2answer.org/advanced.php#theme-advanced
In your theme's qa-theme.php file, override the main() method by using code which is something like the below:
<?php
class qa_html_theme extends qa_html_theme_base
{
function main()
{
if ($this->request=='glossary') {
?>
Your custom HTML content goes here
<?
} else
qa_html_theme_base::main();
}
}
?>