For a while, the Substratum theme engine and Andromeda desktop client allowed users who wanted to apply a system theme to their stock Android Oreo device to do so without having to root it, but Google has confirmed that the OMS engine behind that functionality has been closed off. Google confirmed as much in the official Android issue tracker, marking an issue involving installing OMS themes as “will not fix, working as intended”. This happened in the March security patch, which means that any devices with that patch level and up can no longer use Substratum without root. Obviously, this also applies to Android P. Those who wish to theme their device will now have to take the plunge rooting it, which disallows the use of a number of apps such as Pokemon GO, Google Pay, and numerous banking apps. Getting around this requires a backdoor like Magisk, and those are not only difficult to use and dangerous, they don’t always work.
The OMS engine used by Substratum originated in Android 7.1 Nougat, created by Sony as a replacement for the inferior customization offered by its previous RRO framework. OMS was integrated into Android fully and officially as of Android 8.0 Oreo, but Google never intended it to be used by anybody besides manufacturers. To be clear, this means that Google in no way supports users putting custom themes of any sort on their device, beyond what their manufacturer provides, and never has. Theming without root was only possible for the past few months, in the short time between the release of Substratum and Andromeda for Oreo and the time that the March patch hit.
While Google and OEMs want to give users less reason to root with each new device and software version released, Google cited this action as a potential security risk. That supposition is not without merit; Substratum’s rootless, Andromeda-addled version has access to system resources in order to replace them with themed ones, and it’s quite conceivable that a bad actor could create a theme laden with malicious code. Sony did propose many methods for securing OMS while leaving it open for third-party themes, but since the number of users affected by this change is small, Google is highly unlikely to take a second look. OMS may come back in some user-addressable form in the future since it’s not being removed from Android entirely, but if it does, it will almost certainly be under Google’s control for security reasons. In any case, those wanting to theme their devices, for the foreseeable future, will have to deal with all complications of rooting Android hardware.