I spell it theater, regardless of whether it's referring to a stage show or a cineplex. I only use theatre when it's part of a name like Colonial Theatre, Majestic Theatre, etc. I guess same reason why everyone (including Brits and the like) spell Technicolor instead of Technicolour because it's a trademark. I've actually seen British movies that have opening credits like "colour by Technicolor."
Salve, Regina, Mater misericordiae
Vita, dulcedo, et spes nostra
Salve, Salve Regina
Ad te clamamus exsules filii Eva
Ad te suspiramus, gementes et flentes
O clemens O pia