This could be Apple’s terminology for an enhanced ‘Dark mode’ or something else entirely if the rumor is even true.
We talked to Sonny briefly for clarification and he noted that the source has had some good information in the past. Also, Dickson noted that the theater mode very well might be enhanced version of Dark Mode. He added that the codename for iOS 10.3 is ‘Erie’.
Apple often seeds pre-release versions of iOS updates to carriers for pre-validation so beyond internal Apple employees, carrier employees often have access to pre-releases of iOS software. This information is fairly specific so we’ll find out on January 10th if the rumor has any validity.