Substratum is a rootless way to install custom themes on Android Oreo. When it burst onto the scene around September of last year, it instantly became a crowd favorite; people loved the ability to reduce the increasing amount of whiteness Google was shoving into Android. However, it seems like Google wasn't happy with this, as Substratum and custom overlays in general are no longer functional on Android P.
The lead Substratum developers have confirmed that many are seeing this issue. Android P still allows overlays to be installed, but only those installed by the system; it's similar to what the Razer Phone has. This is pretty unfortunate, especially considering that Android P actually incorporates even more white in its UI than ever before. Even those with root access will be affected; an overlay can't be installed even when forced, as the platform certificate mismatch will still present a problem. Framework.jar would likely need to be patched to resolve the issue, and while this is something that custom ROMs would be able to resolve, those without root are out of luck.
The Substratum theme says that it will try to organize a petition so that enthusiasts can speak out about their displeasure with this decision by Google, and that paying customers of Andromeda won't have had their money thrown away since Andromeda framework is still being developed. But even if the cause gains strong support, at the end of the day, fans are still at the mercy of Google.
The team behind Substratum has a petition you can sign to bring back support for custom overlays and the request has also been submitted to the Issue Tracker (only star it, don't comment as that doesn't help and sends emails to everyone).
- XDA Developers