In the comments on "Logical prescriptivism" (5/25/2009), where the inconsistency of "myself" vs. "himself" was under discussion, the fact that it's "thyself" rather than "theeself" came up.
But in addition to 15,869 instances of "thyself", Literature Online finds 19 instances of "theeself", all from 19th century drama or from dialogue in 19th century novels. And the authors include Dickens, Trollope, and Twain.
From Nicholas Nickleby (where there are two other hits):
"Brout thee!" replied John. "Why didn't'ee punch his head, or lay theeself doon and kick, and squeal out for the pollis? I'd ha' licked a doozen such as him when I was yoong as thee. But thee be'est a poor broken-doon chap," said John, sadly, "and God forgi' me for bragging ower yan o' his weakest creeturs."
From Framley Parsonage (where there is one other example):
"Come in, Robin postman, and warm theeself awhile," said Jemima the cook, pushing a stool a little to one side, but still well in front of the big kitchen fire.
"Well, I dudna jist know how it'll be. The wery 'edges 'as eyes and tells on me in Silverbridge, if I so much as stops to pick a blackberry."
"There bain't no hedges here, mon, nor yet no blackberries; so sit thee down and warm theeself. That's better nor blackberries, I'm thinking," and she handed him a bowl of tea with a slice of buttered toast.
From The Gilded Age:
"He doesn't say exactly what it is," said Ruth a little dubiously, "but it's something about land and railroads, and thee knows, father, that fortunes are made nobody knows exactly how, in a new country."
"I should think so, you innocent puss, and in an old one too. But Philip is honest, and he has talent enough, if he will stop scribbling, to make his way. But thee may as well take care of theeself, Ruth, and not go dawdling along with a young man in his adventures, until thy own mind is a little more settled what thee wants."
The OED gives theeself as a dialect variant of thyself, but the only citation is
1825 J. NEAL Bro. Jonathan II. 158 Take and read it for theeself.
The OED's entry for self has
3. Following a pron. in oblique case. Obs. exc. in himself, herself, themselves.
with the citation
1576 FLEMING Panopl. Epist. 24 Wilt thou, Seruius, stay thee selfe.
So were there actually 19th-century English dialects that had settled on theeself, whether due to vowel shift issues or as a residue of the himself, herself, themselves pattern? Or were Dickens, Trollope and Twain just unreliable observers of the variants that they assigned to their characters, at least in some particulars?
The second option seems more likely to me in this case, but I could well be wrong. This is a minor point, but perhaps some reader who knows more about than I do about the history of (variants of) English can help out.
[Update: searching for "thee self(e)" as two words turns up a few other hits, including an anonymous 1561 play:
It is my office to bryng the elect of God to Iesus Christe: and therfore my deare brother, repose thee self in me, for I do assure thee that Iesus Christ doth wyllingly accept thee amongest his.