A theorem can be an absolute. For example: x -> x (read: if x is true then x is true). This is a theory (because it is a true proposition) and it is absolute (in the sense that there cannot be a system of derivation in which it is not true). This is a math example, but it's the simplest way to show that theorems can most certainly be absolute.
For another example, one could argue that the entire sensible universe is a mirage and that really only Scientist McDougall exists, but he doesn't know it. Scientist McDougall could still create the theorem: "If the universe I perceive is not a mirage and really exists, then if I drop a bowling ball and nothing is in it's way, it will fall down." This is both a theorem and an absolute. We can see this because statements of the type "If X -> Y" are true under if X = True & Y = True, X = False & Y = True, X = False & Y = False. The only case in which it is not true is if X = True & Y = False. But since this last scenario is not the case, then Scientist McDougall's statement is indeed a truth.
All science is founded on this and a few other basic assumptions (i.e. I'm not constantly hallucinating an imaginary world). We just don't both to open every single scientific proposition with "if it is true that the entire world is not an imagination created by the demon in my head, then..." because, well, it'd be a waste of paper and time.