The final release of Android O may bring about a feature that's long been a staple of custom ROMs and a favorite among modders – support for custom themes. Seeing as how this has been one of the major incentives to get your phone rooted in the past, it only makes sense that Google finally implements this functionality into Android. Nothing is set in stone at this point, so don't get your hopes too high, but we noticed an interesting new option in the "Display" settings page in Android O Developer Preview 2 on the Pixel, which wasn't there before. It's called "Device theme."
XDA, this could be in reference to Sony's Runtime Resource Overlay (RRO) theming framework, which very well-developed and still used by some a number of popular Android ROMs. There is no concrete evidence at this point to confirm this, and Google might have implemented the feature into this developer preview solely for testing purposes."Device theme" currently has two options, "Pixel" and "android.auto_generated_rro_", although "Pixel" does nothing at this point, and the OS automatically reverts to the other option. What's more interesting though, is the "_rro" part in the name of the first setting. According to
Still, it's not too out of the question that we'll see expanded theming options in the final release of Android O. If Google is indeed building on RRO, this will bring even more variety to the table, since there's a ton of compatible themes out there and developers are already familiar with the framework.