Not sure if you're being ironic, but in science the word Theory does not mean the same as in common usage (like in "I have a theory").

In Science, such a thing is called a conjecture, and if a conjecture is not dis-proven (i.e people try to disprove by experimentation and fail), and it makes predictions that can be observed, then eventually it becomes a theory. At that point it can be treated as true, because it works - even if it only explains a small part of nature, if it works there, then it's a theory.

If someone comes along and proves the theory of relativity wrong (like, try this experiment, see how it breaks the theory) then new conjectures will need to appear and maybe one will become the new theory.

Flat earthers have run experiments to try to disprove their conjecture, and actually DID disprove their conjecture, but rather than accept it, they reject the experiment instead.