Evolution can be seen in pesticide resistant insects in farming: When a farmer sprays his crops with pesticide, the resistant individuals will have a better chance of reproducing. This causes some pesticides to become ineffective. Therefore, evolution is an observable phenomenon.
The Theory of Evolution is a theory. A theory is a system of ideas. Therefore, the Theory of evolution is a system of ideas. Ideas are not observable. Therefore, the Theory of Evolution is not observable.
Your proof that evolution cannot be observable is based on it being a theory. Your proof that it is a theory is that it cannot be observed. This is cyclic reasoning, thus, begging the question.
Appeal To Belief
The premise states a false definition, or a common belief of the definition attributed to the word "theory". A scientific theory is a well-substantiated explanation of some aspect of the natural world that is acquired through the scientific method and repeatedly tested and confirmed, preferably using a written, pre-defined, protocol of observations and experiments.