The Oxford English Dictionary (OED) records themself from the 14th century. It doesn’t have a separate entry of its own, but a note at the entry for themselves informs us: in standard English themself was the normal form to c1540, but disappeared c1570. Themselfs, themselves appears c1500, and became the standard form c1540.
So for around 150 years, themself (though ending with the singular suffix -self) was considered to be correct when used to refer to a plural subject. A little more OED-delving shows that a similar situation existed when it came to first person plural reflexive pronouns. The form ourself is first recorded in the 14th century, when it was an accepted usage. There must have been a move towards pluralizing the singular suffix –self to –selfs or –selves for plural reflexive pronouns in the early to mid 16th century, when the forms ourselves and themselves first appeared.