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.