In civilized societies the mores tend to take the form of law though laws and mores always coincide. On the one hand the law may prohibit behavior not in the mores in which case law remains generally ineffective.
Prohibition is a common example; untouchability is another. The consumption of intoxicating beverages is prohibited by law and sanctioned by mores.
Child marriage is legally forbidden but it often takes place. On the other hand all the mores are not the part of law. They are well known and too widely respected to require formal enactment or they are personal matters which are not fit subjects for law.