Add class to modal

Problem/Motivation

I tested the module and I find it really very complete. I only miss the possibility of adding a class on the modal. We have the twig template but I would like to add a class on the modal. We can modify the height or the width, I imagine that we could add a configuration field to add this class.

The problem is that I don't understand how this modal is created. I'm used to opening them with the form API, ajax commands, etc. but here I only see js for closing.

Proposed resolution

Is it possible to explain to me how this modal is generated? I could then make a merge request to add the class field.