aospUI Gray theme matches exactly the colors and UI that Google made for Android M second preview and N, extended on more apps, it's "the old" O/Pixel Dark Mode.
Now with optional Google Pixel colors and Android O style!
The theme installs through an app that this app will link you to, IT REQUIRES ROOT (except on OMS rootless, Oreo and on Samsung Touchwiz Nougat/Oreo system).
The theme supports the following systems:
-AOSP/Custom/Google ROMs from Nougat, up to Oreo 8.1;
-OxygenOS Nougat and Oreo;
-Samsung Nougat and Oreo stock system (including S series, Note series, A series and J series).
On Samsung Nougat notifications can't be made dark.
Other manufacturers' ROMs aren't guaranteed to be compatible (the more different they are from AOSP or Google ROMs UI, the more they're likely to not be compatible.
In case you end in bootloop flash the rescue zip that you find in /sdcard/Substratum/ (the substratumLegacy one if your ROM is RRO. Only most custom ROMs have OMS, the rest has RRO) and ask for refund, also by e-mail if you need.
Themed elements list (70+ apps): http://txt.do/dlvhx
Thank you all for supporting me. :)