Customization of free version

Hi,
Just want to check if customization is allowed in free version? I mean I have free version running so just want to check if I do some manual coding and customize it as per my requirement, is it allowed or I have to pay some fee for it before doing?
Just want to check if customization is allowed in free version? I mean I have free version running so just want to check if I do some manual coding and customize it as per my requirement, is it allowed or I have to pay some fee for it before doing?