Is there a way to make a custom webmail interface for my mailserver

I am currently trying to work on a project which requires me to setup a mail server (not to difficult) and a webmail client. However, I do not want one of the generic webmail interfaces, I want to make my own custom design. I have found open source webmail clients such as rainloop etc, but was wondering what goes into making a webmail client from the ground up? Any answers are appreciated to give me a direction in where to start.

Thanks.