We already have a theme per site. We would like to create a common/shared theme and configure for few SXA sites.
Just create it like you normally do.
Then if you want to have it available on other sites add it to
Compatilbe themes field (notice that I added Wireframe which is default theme)
If this will be common theme it would be good to store theme classes in a single place (if there will be any, I assume there will be some).
Then you might want to create base site for that, here are details.