Is a person held accountable, Islamically, for taking a sapling from the street trees to adorn his house with it, and if that is not permissible, what should he do with the trees he has already planted?

It is impermissible to take the fruits of trees planted in the street, whether for ornamentation or other purposes, without the permission of their owners, be they a public or private entity. Taking without permission is considered an encroachment upon the property of others. If something was taken without permission, one must repent and seek forgiveness, ask for forgiveness from those responsible for the trees, and if they do not grant permission, then what was taken must be returned.