On a recent morning in Manhattan, the veteran costume designer Molly Rogers, a cheerful blonde with a honeyed Southern drawl, was standing outside Saks Fifth Avenue, waiting for the store to open. She needed to return a pair of Stella McCartney heels she’d bought, about a week before, to wear to the New York première of her latest project, “The Devil Wears Prada 2.” They had a gold chain across th...