In science, a theory is a system which incorporates proven facts and evidence and is therefore adopted as a valid way of thinking and explanation.
For instance, tracing the roots of the Etruscan language to one or another equally possible alternatives; one has one theory, the other another theory. But both have got to have reasons behind them which are valid in terms of analysis, linguistics, etc.
Creationism and flat earthism are not theories, because both have long since been disproven. Hence they have no validity.