You can use whatever you like on your backend. This /is/ just a straight-forward CSS/HTML/JS frontend, however the theme's author has helpfully (un- in your cas

You can use whatever you like on your backend. This /is/ just a straight-forward CSS/HTML/JS frontend, however the theme's author has helpfully (un- in your case) provided the theme's basic structure a set of PHP files such that you can use it as a template without breaking it apart yourself.