The current Customize from SpicyChicken, the app's original developer, works for some on 1.13, others not. There is, however, a modified version of Customize on Installer from Skylar that works on all 1.13's.
Just tried it again and nothing. The first screen with Spicy chicken logo apears for about 20 seconds and then it just goes back to the home screen. Im restarting right now to see if that works. Are there any other programs needed in conjunction with this to get it working?