i just checked the spelling on all my favorite music venues in so cal usa.
greek theatre los angeles, the el rey theatre, the roxy theatre, gibson amphitheatre, john anson ford amphitheatre, orpheum theatre, ventura theatre.
i've also subscribed for years to our local canyon theatre guild.
i actually couldn't find or think of a "theater" within 100 miles!
just to stay a bit on topic, none of the above have "theater" in their domains either...