Create website using open source classified ad website code

I would like to have a website made from the open source of : [login to view URL] I have some features/options id like to be added to the website but I don't know if this open source let you modify it heavily. I would also like the design to be changed into a prettier design that is responsive. When applying for this job answer this question : Do you think this opensource code can be modified and in what coding language and with what would you modify it to suit my needs?