PORTOLA VALLEY -- If the architecture of Silicon Valley calls to mind the tilt-up office parks that surround its freeway exits, you might have missed Beverly Thorne’s jagged, steel-framed house nestled in a cliff overlooking the South Bay.The airy home is one of the most stunning of an estimated 30,000 mid-20th century Modern houses dotting the region from San Jose to San Francisco that are attracting new admirers among monied young software engineers with an affinity for clean designs.“They call us the underground ‘Midmod’ society,” said Monique Lombardelli, who helps discover the homes for the tech elite. “A lot of my clients are Facebook buyers, Google buyers. They love design.