The legal definition of theft is the generic term for all crimes in which a person intentionally and fraudulently takes personal property of another without permission or consent and with the intent to convert it to the taker's use (including potential sale). If you find something which does not belong to you and you keep it, it is theft.
Does that then apply to this great country in which you live and reap the benefits from which have invaded other lands and removed the indigenous people as the owners of those lands?