aospUI Gray theme matches exactly the colors and UI that Google made for Android N second preview, extended on more apps, it's "the old" O/Pixel Dark Mode.
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/Pie system, where you will have to buy an add-on made by Substratum developers).
The theme supports the following systems:
-AOSP/Custom/Google ROMs from Nougat, up to Pie;
-OxygenOS Nougat and Oreo (+Pie without Settings and some details);
-Samsung Nougat, Oreo and Pie ( One UI ) stock system (including S series, Note series, A series and J series).
On Samsung Nougat, notifications can't be made dark.
It is recommended to connect your phone to adb on your PC before installing the themes unless you're rooted, because it can recover your phone from eventual bootloops without wiping.
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/, if you have custom recovery (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
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