How to theme an entityreference field?

Hi,
thank you for this great module.

I would like to know what is the recommended way to theme an entity that only exists as a field of a node.

In my case I have a cutom module that defines a custom entity type X, then it uses entityreference to link a list of those entities to a node type N.
So I would like that the pages of N just render the instances of the entity X following a template file I write.

What is the best way?
I see this module exposes different formatters, but I think it is not enough...

Can you help me?

Thanks