If you have preordered ff7remake digitally, then go to psn store and search for the ff14 free trial. Try to download it, should come up with an error message stating something along the lines of you cant download this as you already have purchased the game. This in effect adds the free trial to your library which is one of the pre-reg to download the theme. You should then be able to download the theme and use it. Best of luck (it worked for me anyways)