Android Oreo may soon get support for custom themes without rooting

Image source: XDA
The developers over at XDA found a way to take advantage of OMS (Overlay Manager Service) that is now integrated into Android 8.0, and to get Substratum themes up and running on it without any need for rooting or otherwise tinkering too much with the OS.

Image source: XDA
The method currently involves escalating permissions through ADB, with the only real drawback being that these permissions are lost after reboot, which means you'd either have to stick to the theme you've applied or punch in some ADB commands again.
If you're interested in this development, check out the full read over at XDA (linked below).
source: XDA
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