As you may have heard, there is no 'theory of gravity'. There is however, a 'law of gravity'. The existence of gravity is not in doubt. IMO the existence of evolution is not in doubt. The mechanism of evolution by natural selection is a theory, and requires a faith in materialism, since it assumes that direction is not a possibility. This has not been proven, and probably can never be proven. It requires a proof that God does not exist.
There is no Board of Scientific Laws that deems some things theories and some things laws. There is only differing levels of certainty regarding theories.
There most certainly is a "theory of Gravity". In fact, there are several. The one we currently adopt is currently being hammered by the fact that it seems to fail for outlying stars in rotating galaxies.
We thought, until very recently, with high confidence, that the continents were fixed on the earth. No reputable scientist thinks that there is closure on any "law" of science, such that we no longer need think about it critically.