This question already has an answer here:
When a user does not fill his "about me" section in his profile, the following sentence appears:
Apparently, this user prefers to keep an air of mystery about them.
To my knowledge (which are restricted I admit), it should be
Apparently, this user prefers to keep an air of mystery about him/himself.
I'm pretty sure that I'm wrong, however I really don't understand the use of plural in "them".
Could you please explain me the underlying grammar rule that justifies such a term? Also, I'd be happy to know if replacing them by him or himself would be correct.