Which reflexive or intensive pronoun should I use for an unknown person (himself, herself or itself)?

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