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
There’s no such thing as “correct” grammar. Different ways of speaking (or writing/typing) can certainly make people view you differently. For instance you might be seen as more or less smart or educated or affluent depending on your way of speaking. But that’s pretty much just stereotypes.
I watched part of an MIT OpenCourseware course on linguistics once. (Very good and recommended, despite the fact that I haven’t finished it.) At one point the professor told the students that “we’ll speak no more about prescriptive linguistics except to mock it.”