Each person will have to decide for theirself

[FalaGringo]

is "Theirself" and "Themself" grammatically acceptable now?

1. "People know themselves well."
2. "Someone definently talks to theirself."
3. "Someone will do it theirself."
4. "One knows oneself well."
5. "Everyone" wishes they had figured it out theirself / themselves."
6. "Each person will have to decide for theirself."

I've always felt that "Someone" is singular and "Everyone" can be both plural and singular.
What are your thoughts on the matter?.

 

[5jj]

Theirself and theirselves are incorrect in standard English.

Themself is becoming accepted as a reflexive/emphatic form of singular they.

 

[Jemima23]

Not a teacher

Here's a quote I found on the subject: "What does theirselves mean? Theirselves is a dialect form of themselves that is widely shunned and rarely crops up in written English.It is slightly more common to hear spoken than written, but even in spoken English, the use of theirselves is a marker of poor English."