pronouns - Can "themselves" be singular?

(This is probably a duplicate, but I couldn't find what I was looking for through search.)

Say for example you have the sentence:

He is referring to himself.

You can replace "he" with "they" (singular), but you can't replace "himself" with "themself" because it isn't considered a real word.

So, would it be "themselves"? Can "themselves" be singular? If not, what would you use instead?

They are referring to __________.

^{(Singular "they")}