I think there is confusion about the word ‘theory’.
https://www.gov.uk/book-theory-test
“Mirror, signal, manoeuvre” or “red traffic light means stop” are not opinions or proposed explanations. They’re rules of the theory of driving.
People use ‘theory’ when they mean suspicion, opinion or proposed explanation.
– “Theory – a formal statement of the rules on which a subject of study is based or of ideas that are suggested to explain a fact or event or, more generally, an opinion or explanation“.
This confusion is often promoted by creationists; “evolution’s just a theory”.
“Theory of music”?