"I had one fine composer tell me this once:
"Using theory is like putting the cart in front of the horse. That is, theory is a collection of patterns caused by music throughout history. These hard and fast "rules" that we have in theory create music of a particular style."
Whoever started this "hard and fast rules" crap relating to music theory is on my hit list. I'm so tired of hearing it being conveyed this way. So, I'll just add that there are no hard and fast rules in music. If you want to evoke a mood in the listener that a particular stylistic pattern happens to help with, then theory is immensely useful. This "putting the cart in front of the horse" business is such a massive over-generalization of what we do when we use theory to aid us in pulling something off in our music. Theory can be part of the compositional process, and there's NOTHING wrong with that.