It is a bit disingenuous to use the argument that evolution is “only a theory” to try and discredit it. While it is a theory it is not a theory in the everyday sense of the word that you defined. The American Association for the Advancement of Science writes “A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. Such fact-supported theories are not “guesses” but reliable accounts of the real world.” In science the everyday use of the word theory would translate to a hypothesis. It’s always confused me that people try to discredit evolution using this argument. You don’t often see people going around trying to discredit things like the theory of gravity, cell theory, germ theory of disease, or tectonic theory. People seem to be fine with theories right up until evolution is brought up, then it’s “only a theory”. Unfortunately unlike other things we don’t get to pick and choose with science.