Is it breaking the law to steal marijuana plants from neighbor backyard?

Yes. It's not illegal to steal an illegal substance, but it is illegal to posses an illegal substance, which would occur at the exact moment of the stealing. It is also illegal to trespass on property you do not own.

However, it's a moot point, as your neighbor will never press charges because he has more to lose by doing so than you do.

So legality of stealing ur neighbors plants: illegal

Consequences of stealing ur neighbors plants: angry neighbors?