There's no such theory, and exposure to light brightness has nothing to do with myopia. Being outdoors causes less close up focus, which slows the progression of myopia.

Not "light brightness," whatever that's supposed to mean, but UV light.

Since I learned that excess UV radiation burns the retina, I've started always wearing sunglasses outside. Should I stop?

The way I understand it is that it only matters while the eye is still developing, so if you're over 10 it's too late anyway. The thing with UV more generally is that we've obviously evolved with them, so while they are dangerous in some respect we also depend on them. In this case, protecting your eyes too much early is probably detrimental, but not protecting them later on probably is as well.