How do I update the class list dynamically (or get the id of the modal)?

[Brozerker]

I'm implementing a fullscreen button where, upon button click, I'm trying to add or remove a class to ModalInstance.Options.Class.
I've verified that the property is being updated, but the dom is not. How can I update the class list so that it updates the dom?

Alternatively, how do I get the id of the element so that I can update the class list in javascript?
I tried using ModalInstance.Id but that doesn't match the id that's on the element.