The theory of evolution is a valid theory because it is in fact testable despite what The Theory Of Evolution is not a valid theory states. The test for the theory was stated by Charles Darwin in On The Origin Of Species.
The test is whether there exists a biological structure which cannot have come into existence by a series of smaller evolutionary steps.
Individuals who believe that there do indeed exist these structures call their theory Irreducible Complexity. They examine complex structures such as the eye, and brain but also many other structures to check whether it seems possible for it to have come from the process of evolution.
That makes it a hypothesis according to science, which defines a hypothesis broadly as any idea which there exists a test/tests for which would falsify that idea.
A theory is normally a collection of multiple hypothesises to explain something, and which has been tested over and over and repeatedly failed to be falsified. That does not make proven however, The problem of proof says that nothing in science is ever scientifically proven no matter how many commercials state otherwise. However it does mean the idea is well supported by testable evidence, that the tests done are repeatable and the same or very similar results will be found if those tests are repeated. As such theories are assumed true until new evidence comes along which challenges that belief.
Evolution meets all the conditions for a theory as defined by science. It is therefore a theory.