Gravity isn't a scientific theory. Gravity Theory is a scientific theory. Gravity is a word given to the gravity experience that happens to all people all around the world all the time.
Yes, gravity is a thing that happens. We didn't know what it was until we had a theory to describe it. Crazy as it might seem, some people actually thought it was a work of "God". But now we know what it is and why it happens, we can apply it to explain many other things, such as the motion of the planets.An engineer can make a scientific theory. But that doesn't stop him from engineering without using a scientific theory. He might use a non-scientific theory. Or, if it's simple enough, he might barely use a theory at all.
No, you're thinking of wizards. Engineers use physics. Wizards use made-up magic stuff.