Steppenwolf Theatre Company is offering a first-time-ever tour of its scene shop to those interested enough to fork over $250 for the company’s “Behind the Curtain” benefit. Though Steppenwolf is known primarily for ensemble acting, its shows owe as much to designers as performers. Case in point: the troupe’s ballyhooed August: Osage County, which ran on Broadway for two years following its world premiere here in 2007. Among the show’s five Tony Awards was the one Todd Rosenthal received for his scenic design.