Stack Exchange profile: “This user prefers to keep an air of mystery about them” [duplicate]

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.

Solution 1:

When the gender of the person is unknown, 'them' can be used in the sentence. Though, 'him/himself' is also a correct.

Please refer this blog that talks about usage of plural pronoun as singular