Exquisite Curved Sofa. Mid century style that introduced Americans to sectional, modular seating in the 1940's, when it became a furniture industry phenomenon, had in the last five years found a new appreciation among collectors of midcentury-modern furniture, especially for his upholstered pieces and cabinetry. Mid century designs were often characterized by understated modern lines relieved by elegant details like delicate hardware, warm finishes or unexpectedly bright colors. The 1950's were balanced design with decoration because customers, easily bored with the academic purities of modernism, were ever eager to ''horse things up.''