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
It also doesn’t help that the third person feminine is ambiguous. There’s often no distinction between the accusative “her” and the possessive “her” (except when the pronoun appears in a different part of the sentence and becomes “hers” - fuck I hate English), so it could be interpreted as fitting either rule.