What are the requirements for a supplemental brief in a personal injury case?

Appointed counsel filed an opening brief that sets forth the facts of the case and asks us to determine whether there are any arguable issues on appeal. (People v. Wende, supra, 25 Cal.3d 436.) Counsel advised defendant of the right to file a supplemental brief within 30 days of the date the People filed their opening brief. Defendant filed a letter with this court in which he indicates he is sorry for his conduct and asks this court for "a lighter sentence" so that he may be a father to his child. Defendant does not, however, claim any error.