THEIR, pronoun pronominal
1. their has the sense of a pronominal adjective, denoting of them, or the possession of two or more; as their voices; their garments; their houses; their land; their country.
2. Theirs is used as a substitute for the adjective and the noun to which it refers, and in this case, it may be the nominative to a verb. 'Our land is the most extensive, but theirs is the best cultivated.' Here theirs stands as the representative of their land, and is the nominative to is.
Nothing but the name of zeal appears
'Twixt our best actions and the worst of theirs.
In this use, theirs is not in the possessive case, for then there would be a double possessive.