huh. I found this in the [online] Compact OED:

themself
• pronoun third person sing. informal used instead of ‘himself’ or ‘herself’ to refer to a person of unspecified sex.

— USAGE The standard reflexive form corresponding to they and them is themselves, as in they can do it themselves. The singular form themself, first recorded in the 14th century, has re-emerged in recent years to correspond to the singular gender-neutral use of they, as in this is the first step in helping someone to help themself. It is not generally accepted as good English, however. [EA]