right way to manage themes with multistore

[Beeta]

Hello,

I created a second store that is going to be used by my wife with her's handmade creations.

The main shop is dedicated to my father in law products instead.

I would like to use the default theme in each store, need I to duplicate the theme?

or I just need to create child themes? how they works? they are "virtual" themes that inherit from the parent one?