It's "themselves" unless you want to say "himself or herself" every time.
A side note: Although this is how I reference and understand referenced a person of unknown gender, there have been disputes about the usage of gender-neutral pronouns, especially in recent years. However, you would be understood using "themselves":
Use themselves as the reflexive/intensive pronoun to refer to an
indefinite gender-neutral noun or pronoun that is the subject of the
sentence and avoid themself.
I would rather state the other voice regarding this, too:
Although some current dictionaries, for example, The New Oxford
Dictionary of English, state that themself has re-emerged in recent
years when used to refer to a singular gender-neutral noun or pronoun
("themselves" remains the normal third person plural reflexive form),
they label it as "rare" or "disputed" or "not widely accepted in
standard English".
The references are from Department of Justice of Canada
Intensive: I need to run a mask detection, but I just need to use someone's head, not the person himself/herselvf
– Maf Dec 07 '20 at 09:59Is that normal to collocate
– Maf Dec 07 '20 at 10:18someonewiththemselves. Should we also usesomeonefor plural?"If people want to kill others, they should do it themselves."
"I need to run mask detections, but I just need to use people's heads or faces, not the people themselves."
– Yosef Baskin Dec 07 '20 at 19:31