A stylish woman feels free to mix and match. Wear Hermès with J.Crew. Tuck Italian silk into French denim. Pair an evening bag from your former ­husband with jewelry from your lover. In 1965, opera star Maria Callas attended a London soirée carrying a yellow gold and diamond basketweave clutch that her ex, Italian industrialist Giovanni Battista Meneghini, had purchased from Van Cleef & Arpels eight years earlier. (“These bags were very à la mode in the 1950s and 1960s,” says Catherine Cariou, the company’s ­heritage director.)Callas also wore earrings and a bracelet made of round cabochon-cut coral surrounded by rings of round diamonds, cultured pearls, and turquoise set in yellow gold. They were special-ordered from the same ­jeweler by the soprano’s then-boyfriend, Greek shipping magnate Aristotle Onassis (whom she took up with while still married to Meneghini). So w