In all theaters with backdrops in the chest (the soon-to-open Shard theater, Therenborough, and Ratha), you should now be able to look in the chest once you're on stage and (better still) look at the backdrops in the chest to see what backdrops are available, as well as how to use them.

This was not an issue for the Crossing amusement pier and Muspar'i theaters - as the signs there direct, look on the curtains once you enter the stage.

-V.

"Reject me not, sweet sounds! oh, let me live,
Till doom espy my towers and scatter them.
A city spell-bound under the aging sun,
Music my rampart, and my only one."
-Edna St. Vincent-Millay