The word "theory" doesn't change meaning depending on the preceding word. It means...

"A set of statements or principles devised to explain a group of facts or phenomena"

A theory can be lacking in evidence, or supported by evidence. I mean, 9/11 is a "conspiracy theory", and there is evidence to support such a theory. Granted, it's not conclusive evidence, but the word "evidence" is another word that is misused. Evidence is not proof.

The "evidence" that flat earthers present can be conclusively proven to be bollocks, but that doesn't mean they're not presenting evidence. It just means their evidence is shit.

A "conspiracy theory" is not "a conspiracy that is lacking evidence", which you seem to think it is. It's better to say "a theory that is not widely held to be true".