Same place TCK.
PHP Code:
$article_content = "HTML, JAVASCIRPT OR WHATEVER HERE.";
Although be careful, if you must use double quotes, put a \ (I think... if it doesn't work try the other one XD) or instead of the double quotes use a single ' . The script should still work and everything.
You can also change the entire HTML of every page by accessing the template.html file under the templates folder.