theirselves/themselves

No pronoun as 'theirselves' or theirself'. The appropriate pronoun is 'themselves' not 'themself'.

Themselves is the standard reflexive pronoun form which corresponds to the plural forms they and them :e.g. They are talking to themselves

NOT They are talking to theirselves/theirself. 

It is also used reflexively as the direct or indirect object of a verb or as the object of a preposition: The boys prepared themselves for the trip;

Watch this the next time you want to use the pronoun.