The origin of the HTML is qa-theme-base.php. However, to customise the HTML you should edit the qa-theme.php file in your theme (e.g. qa-theme/Default/qa-theme.php).
Where you want to change some HTML, copy the appropriate function from qa-theme-base to your qa-theme and change the HTML.
You can also change just the CSS by editing qa-styles.css in your theme folder.