I find myself often winging it with “themself/themselves” and it seems to be like themselves is always colloquially correct when there are multiple preceding nouns you’re referring to…
Otherwise if there’s only one antecedent or whatever, its themself
Be gentle haha
I’ve seen a few people prescribing “themselves” even for the singular, and interestingly enough Wiktionary mentions that “themself” is sometimes proscribed. Based on that I’d argue that the linguistic community didn’t “settle down” on the rules of when to use one or another.
That said, personally, I use it like you do: -self in the singular, -selves in the plural. Same deal with [our|your]+[self|selves].
I don’t care if it’s not correct - I use “theirself” and “theirselves.” It jibes with “yourself,” “myself,” and “herself.”
“Himself” is a frustrating outlier, but I do know at least one person who says “hisself,” and that’s enough precedent for me.
Weird - I never noticed that the third person reflexive forms typically use the main case (him/them) as a basis, while the others use the possessive (my/thy/your/our). No idea on why this difference.
That said “hisself”, “theirself” and “theirselves” don’t sound bad to my ears.