Retina support comes from using size-agnostic techniques - such as vector images and CSS-based design elements, instead of standard pictures. So, template developers may choose to use this technique or not, so 3rd party templates may or may not be supporting "retina" resolutions.

Another way to hack this is to provide high-resolution imagery and load it for retina-displays, but again, this is just a hack, that won't help with page zooming.

Basic idea of retina is that it's a high-density see more displays (normally over 200ppi) where pixes can't be seen by human eye, and hence "native" resolution is no longer a limiting factor.

It's a major shift in web graphics design world and since it's only started to happen, a lot webmasters are still confused.