Modal Customer not showing on mobile and CSS edit

I’m using POS Pro and almost everything works fine.

I have 2 main concerns that I want to work on it:

1. The modal to register a new customer doesn’t appear on mobile. It opens, the fields seems invisible (they can be clicked, but they don’t appear). It seems they are hide behind something.

2. I cannot edit the CSS of the POS. How can I do some CSS customizations?

By the way, it looks simple, fast and great.