Lately, Samsung and HTC came up with theme engine support preloaded in their latest flagship smartphones. Now, it appears like this theme thing is catching on the Android operating system. Google has apparently added limited theme support in the Android M iteration.
It is the RRO (Runtime Resource Overlay) that is the software used by Sony to theme up ROMs in its Xperia lineup of devices. This has been merged with the AOSP. Though this theme engine support is an addition to the other goodies included in Android M, it requires a root as the others.
As Android M will have limited theme support, users need not opt for custom ROMs to change the look of their devices. All they need to do is root the device and use an app similar to Layers Manager to apply some themes. This should be a great option for the users of stock ROM with minor tweaks such as Xposed modules.
As per a Reddit user who has tried a few themes using Layers, all the themes were functioning fine without any glitches. RRO included in Android M definitely hints are better things bundled in the build.