Yes. California has a Felony Murder law codified into law. The event of a homicide during the commission of a felony (Robbery or Mayhem on Theo's part), even if he did not commit the murder himself, would mean he can be charged with 1st Degree Murder. It was likely, and did happen, that murder/lethal force would be used during the crime, and Theo would or should have known this.
A 1987 SCOTUS ruling in Tison v. Arizona would have bolstered any such charge in the following year. Specifically:
Held: Although petitioners neither intended to kill the victims nor inflicted the fatal wounds, the record might support a finding that they had the culpable mental state of reckless indifference to human life. ... Because the Arizona Supreme Court affirmed these death sentences upon a finding that the defendants "intended, contemplated, or anticipated that lethal force would or might be used, or that life would or might be taken," the case must be remanded.
SCOTUS ruled that the use of Felony Murder rule on someone who did not do the killing themselves, due to the circumstances of the planned crime, can be valid.
The Tison brothers only escaped the death penalty afterwards because of their age at the time, having been under 20.