singular they - What is a proper gender-neutral form of himself or herself

I am thinking "themself" is a very old deprecated way of saying this. What would be more proper?

An example would be:

One does not simply build Rome themself.

It just seems awkward with "themself."

Answer

Singular they is a usage hallowed by time, and, since reflexive pronouns are inflected on both the pronoun root and the -self/-selves reflexive suffix (e.g, my[sg]-self[sg] vs our[pl]-selves[pl]), the correct reflexive for singular they is themself.

Dictionaries, by the way, are not reliable sources for grammar, just as grammars are not reliable sources for lexical meaning.