There is a clear distinction one needs to make.
Evolution as a process can be clearly demonstrated, both in logic and in reality.
It is very easy to demonstrate that if you have something that creates copies of itself (by some process) and there is occasional variation (errors in the copying process from one perspective), and there are different environments available, then the variants will have different survival probabilities in those different environments.
That is simple, and easy to show.
That is evolution in a nutshell.
And as systems build in complexity, it rapidly gets profoundly and recursively more complex.
So in this sense, evolution as a process is a demonstrable fact, beyond any shadow of reasonable doubt. We can see it happening, here and now.
What is theory, is the assertion that this process alone is responsible for all the life we see on this planet.
There were no video cameras on earth 4 billion years ago, to allow us to see exactly what went on.
All we have is a few very old rocks (and not many of those, as the earth’s surface is essentially a thin skin on top of boiling rock and is constantly turning over – like the skin on pot of gently simmering porridge). We need to make a lot of assumptions about the sorts of conditions that created those rocks in order to make any sense of what we see in those rocks. The further back in time, the less actual evidence we have and the greater the probability of error in interpreting what data we have.
Yet when one looks at the structure of the molecules of life (and I started my training as a biochemist 45 years ago), then the balance of evidence is very strongly in favor of the theory that all life on earth is the result of the process of evolution by natural selection acting over some 4 billion years.
And that has to rely on some very indirect evidence, as there is no direct video evidence.
Thus we say that the hypothesis that all life on earth is the result of evolution by natural selection acting initially on simple replicating molecules is a theory, and the process of evolution (which we can observe here and now) is a fact.