Why is it not theatrum?
in theatrum would more imply movement -> "into the theater". E.g. Histriones in theatrum eunt -> The actors are going into the theater.
in theatro implies the location where the actors are (or with another verb, where the action takes place).
Is, Histriones in theatro sunt, wrong?
No, it should be fine. If it was rejected just report it with the flag the next time you come across it.
Same question marked wrong Aug 21