I personally called Moreinu v’Rabienu HaGaon Harav Shloimele Rosenbaum, the haliger Shaputzniker Rebbe of Tzfas, shlit”a, and put to him the following question:
“Is there any reason that wearing sunglasses would not be considered tznua?”
His answer, word for word:
“That’s one of the most retarded things I’ve ever heard.”