Part of the problem lies in the various uses of the word, theory. This from dictionary.reference
The problem that people confuse numbers 1 and 6 or 7. The example they give, Einstein's theory of relativity, is a good one. There is NO question about this 'theory'. In this case theory means as it says in #1. It is coherent, it covers a wide variety of cases, it has been verified in the laboratory millions of times. For example, the GPS satellites are so accurate that they have to compensate for the slight fluctuations in time caused by general relativity. Relativity is not theoretical and we don't speak of it any longer as 'in theory'. It has moved to well established theory.
But language is funny and varied. We can also use the word thus: "In theory I could play in the NBA", or "Theoretically, I could get a date with a super model". Creationist like to cherry pick and use THIS form of the word theory when in fact it is not at all connected to Darwinism. The entire book, Why Evolution is True, goes on to give many concrete pieces of evidence that are the foundation, and proof of Evolution. It is a theory but it's not "just a theory".
Interesting that of all the well founded theories in science (gravity, DNA, the atom, etc.) creationist only have a problem with one of them - Evolution. I have a piece about that in an earlier post that may be of interest here.