The box office is entering the “Backrooms.” A24 and Chernin Entertainment’s new horror movie, based on director Kane Parsons’ Youtube creepy series of the same name, is expected to be another hit for movie theaters. It has made $10.4 million in Thursday previews at the box office. “Backrooms” will shatter A24’s record for the company’s largest box office opening of all time. It’s expected to blow past its already monstrous projection of $40 million to $50 million, which would beat Alex Garland’