playSMS - how to customise the web look and feel

Surprisingly, under the /var/www/html, you don't find the index.html but index.php instead and despite that, it is not easy to figure out on how they programmed the html.. based on the "source code", from the browser, I can only identify some of the codes are from

./plugin/themes/default/templates/themes_layout.html

but yet to understand the logic behind