"Themselves" is, at least as far as I learned english, a proper usage in that context. I adjusted "pronoun" to "gendered pronoun" above just in case, in this modern world, the meaning wasn't somehow clear. This feels like an unnecessary amount of pedantry for a simple comment.

> "Themselves" is still a pronoun, and a plural one, at that.

https://www.merriam-webster.com/dictionary/themselves, see "Can they, their, them, and themselves be used as singular pronouns?"

"Someone came into this room. I don't know them, but they took it upon themselves to take everything."

What would be the correct pronouns here in your view?

Not who you're asking, but I would say 'themself' here. I can't see that it's ('themselves' is) not plural.

(Edit: Actually the more I think about that the less I'm sure what I'd say..)

Unfortunately themself is a spelling error according to my browser, and I have to admit it sounds like the most jarring option.

I'm suddenly very interested in the distinction here - as yea themself also works in both of these contexts but themselves feels like a very distinct connotation and not incorrect. I can't point to a specific grammatical rule or learning to base this feeling off though - I don't know why it feels correct it just does.

> A reflexive pronoun is a pronoun that refers back to the subject of a sentence or clause. You can recognize a reflexive pronoun by its second half: they all end in -self or -selves. [...] Only the dual-purpose your* has two reflexive forms: singular yourself and plural yourselves. [...] But if you'd been typing in the late 1300s, themself would have been the default: it was the only version around until the mid-1400s. [...] Eventually, themselves became the only accepted form. But themself never fully disappeared.

https://www.merriam-webster.com/words-at-play/themself

Honestly, while I might say such a sentence, I wouldn't allow myself to write it.

I'm curious what the closest sentence you would allow yourself to write would be.

I'd go with, "Someone I don't know came into this room and took everything," or even, "Someone came into this room. I don't know them, but they took everything." I'm not against a neuter pronoun where it's justified, but there was no need for "themselves" here, and it was worse than several alternatives.

Okay, I guess that answers my immediate sub-question, but it misses the point about the plurality of the pronouns.

> or even, "Someone came into this room. I don't know them, but they took everything."

"Them" would still be plural by your earlier logic though. Unless you're suggesting that you think "themselves" is plural and "themself" would be the singular?

it's pretty common to use 'they' when gender is unknown or irrelevant