Android and customizability are pretty much synonymous. And one of the many reasons why most folks prefer Android over any other operating systems is the fact that Android offers users to tweak and customize it on multiple levels to their liking.
Taking this customization to a whole new level, Android 8.0 Oreo will now support system-wide themes without the need for a root. Theming of third-party applications, custom fonts for third-party applications, and other modifications are also possible now.
For those who are not in the know, system-wide theming was possible on Android earlier too via Substratum theme framework, only you had to root your device before.
But, folks at Substratum seem to have found a way to get these themes work on any device that’s on Android 8.0 Oreo without having to root it. Previous versions of Android, however, doesn’t allow you to do the same since Google had provided full support for the Overlay Manager Service (OMS) — previously known as Runtime Resource Overlay (RRO) to the developers only recently.
Source: XDA Developers