I work at The Simon, it's a really cool set. The theatre has been transformed. I'm guessing here, no inside info, just observations: it's sort of a "world within a world" concept, perhaps? It's stunning and extremely detailed. To my untrained eye (I have an M.A. in Theatre history with a minor in Theatrical Architecture) it appears as if they've ripped the theatre open to reveal to us the world where CATS takes place.
The ceiling of the orchestra is strewn with strings of multicolored lights with bulbs just popping color everywhere during certain numbers. (I am revealing my ignorance here, I've never seen the show before.)
The great ceiling of the balcony now appears to be torn open to reveal a majestic skyline, resplendent with magical pinpoints of starlight and the occasional stray moonbeam, emanating from a gigantic moon which appears at the back of the set. It's mesmerizing.
From President Obama: "Over the years, musicals have been at the forefront of our social consciousness, challenging stereotypes, shaping our opinions about race and religion, death and disease, power and politics."