There's nothing illegal about bringing outside food into a movie theater but the movie theater could deny you entry to the movie theater or kick you out of the movie theater and not allow you back in should you be caught bring outside food or drinks into the movie theater.
Movie Theaters make the most of their money through food sales so they don't want you to bring your own food to the movie theater and they want you to purchase their food instead.
It's a rule set by the private company who owns and operates the theater so the most the theater can do is to ask you to leave the theater or take the food back to your vehicle before entering or ask you to leave and then if you don't leave the theater as requested then you could be arrested for trespassing.