Center City newcomer Shake Shack is getting in the spirit of DesignPhiladelphia, the city-wide celebration of design and creative innovations that’s going on everywhere this week, by hosting Shift Space Design’s Fairmount Living Tiles installation. The installation showcase’s the local design company’s prototype for an easy to use, plant-and-install, one-size-fits-all green roof modular planting tile kit. It's on display through Sunday. But today, beginning at 5:30 p.m., the burger joint will welcome design buffs in for "Concrete by Design Shake Shack Tasting,” and offer two custom-rigged frozen custards (concretes) for free. Both frigid treats were created in collaboration with Nook Bakery & Coffee Bar and La Colombe. Nook’s Fall Look combines Shake Shack’s vanilla custard and a slice of graham cracker pumpkin pie from Nook Bakery, while the other, dubbed La Colombe’s Espresso Session, features chocolate custard, La Colombe Nizza espresso beans, cannoli shells and cinnamon fudge sauce. Both will be on offer until 7 p.m.