I'd guess in most places it's theft. I said it's not theft under adverse possession. Meaning anywhere, including Texas, you'd have to have someone complain about it before it's theft. Because of cultural taboos the odds of people complaining about it are pretty high.
But even in Texas you have entire museums built on grave robbing, so the law clearly varies. Archaeology is the respected science of grave robbery. And to be honest there's enough family plots in Texas that if you dug up some Civil War graves on your north 40 nobody would ever know or likely care. So it sorta depends. The law is clear, but like most property laws it only applies if someone complains.