Evolutionists usually respond by claiming that to scientists, a 'theory' does not mean what it does to lay people, a casual guess. A scientific 'theory', they say, is a large and complex body of evidence and ideas about an important subject: germ theory, string theory, etc.
Both sides are wrong. The creationists are, as they say, 'not even wrong'. They are arguing linguistic trivia, and not addressing any important issue. But the evolutionists are also wrong. Scientists use the word 'theory' in many different ways, just as laypeople do.
Moreover, the evolutionists are wrong in a deeper sense. Evolution is 'just a theory'.
That's because every damned thing we know is just a theory. Science claims no hotline to the godhead. It has no scripture. It has no one who speaks ex cathedra. All we know is the evidence our our senses, and our ideas about that evidence. Anything in science can be proven false. Anything. Well-established ideas—the composition of matter reflected in the periodic table, Newtonian dynamics, evolution—are unlikely to be proven false, but there's nothing that says they can't be.
Could our ideas about evolution be overturned? Sure. And that's a feature, not a bug. Science makes mistakes, because it's conducted by humans beings, who make mistakes. What's special about science, and the scientific culture, is that those mistakes are relatively easy to correct.
Relative to what? To religious mistakes. Because religion makes mistakes, too. And religion corrects them. Religion is ultimately no more stable or changeless than science. Take a look at the early centuries of Christianity. Ebionites, Marcionites, Gnostics, Arians, Copts—a total mess. It took centuries for Western Christianity to figure out what it believed. Then it went through it all again in the Reformation. And it's happening again, in front of our eyes, in the fights over female and gay clergy. How long did it take for the Roman Catholic Church to accept heliocentrism? How long did it take the Southern Baptists to renounce racism? Yet, they finally did.
The advantage of science over religion, as a way of knowledge, is not that one is right and one is wrong. Both are the product of erring humans, all too easily misled by fancy, by romance, by fear, by hope, born unto trouble as the sparks fly upward. Both are sometimes right and sometimes wrong. Science's advantage is that it embraces mechanisms for error to be identified and discarded. It's a lot more difficult when you have to run circles around inerrant holy writ, or a fatwa that can never be rescinded because the Ayatollah who wrote it dropped dead.