Changing POS Layout

I have decided to contribute by creating a new layout for POS using css and js.
It would be great if anyone could share some ideas or any pointers on it. I would like to know what would be the best way to go about it. Which files do I need to change in order to change the current POS without having to touch the core of erpnext.

Also would it be possible to add an external POS into the app.