One of the main problems involved in discussing whether evolution is a fact or theory is the context. There are two which tend to be comingled.
One issue is the evolution of life forms. There are many robust lines of evidence that life forms have evolved from common ancestors. Unfortunately, some have shortened the idea of the evolution of life forms to the single word, evolution. So in this context, evolution is a fact.
The other issue is the means by which evolution has occurred. Darwin's theory is that of evolution by natural selection. Unfortunately, some have shortened the theory of evolution by natural selection to the single word, evolution. So in this context, evolution is a theory.
This was most eloquently stated by Stephen Jay Gould who wrote: "Well, evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts do not go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's, but apples did not suspend themselves in mid-air, pending the outcome. And humans evolved from apelike ancestors whether they did so by Darwin's proposed mechanism or by some other, yet to be discovered." [Stephen Jay Gould, "Evolution as Fact and Theory," May 1981; from Hen's Teeth and Horse's Toes, New York: W. W. Norton & Company, 1994, pp. 253-262.]