By THÉRÈSE MARGOLIS Thigh-high, rhinestone-studded stiletto boots and flowy, silky, floor-length evening frocks made for an unexpectedly elegant look at the Sivuplé and Steve Madden sustainable fashion parade at the NH Hotel Reforma in Mexico City on Wednesday, Oct. 28. The stunning combination of the party-ready, statement-making pairing combined cosmopolitan chic with down-to-earth practicality since bare shoulders plus exposed toes