Well...
Assuming you've included Modalite in your page, you can just add a few HTML attributes and that's it. No CSS involved. You are only provided with a modal-visible class that you can use to stylize the modal when visible, assuming your default styles define its hidden state.
When a click inside the document occurs, the clicked element is checked for 3 attributes:
data-open-modal should possess the value of a modal's id. If an element with the given id is found, it receives the modal-visible class.
data-close-modal means the clicked element's parent tree is searched for an element with a data-modal attribute. If an element is found, the modal-visible class is removed from it.
data-modal is used to indicate what's a modal. If the clicked element has it, it means it's a modal and should be closed, as this is expected modal behaviour. The modal-visible class will be removed from it.