Yes, evolution is just a theory. But it isn’t a theory in the sense that most people use the word. The confusion comes from the fact that theory has multiple meanings. The differences are really subtle, so it’s easy to confuse them.
Definition 1: a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena
The definition above is the one you read in a textbook (that wasn’t written by Creationists) or a scientific journal. There are many scientific theories out there, and we’ve been able to understand them pretty well. For example, our understanding of gravity has given us the ability to land spacecrafts on other planets, place satellites and space stations safely in the sky, and bulls-eye comets from millions of miles away. Pretty good considering gravity is just a theory.
Definition 2: a tentative theory about the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena
Most people who claim that evolution is just a theory are using this definition. The synonym to this particular definition is hypothesis, and that’s really what most people mean whenever they say theory.
From a scientific perspective, most people misuse the word theory by equating it with hypothesis. However, from a linguistic perspective, if enough people are breaking a rule then the rule ceases to exist. So we end up with two irritatingly similar definitions of the same word. If you claim that evolution is just a theory then you are correct. Just make sure you know the difference.