This theme is exactly the one that Google put in Android M second preview and N extended on more apps.
Now with optional Google Pixel colors and Android O style!

The theme installs through Substratum Theme Engine app that you can download here IT REQUIRES ROOT (except on OMS rootless and on Samsung Touchwiz Nougat system) and/or a Layers or Substratum compatible ROM: https://goo.gl/BB5eio

The theme has been tested on many ROMs including stock Google and custom Marshmallow and Nougat ROMs (OxygenOS is supported too it is Substratum Legacy though).
Android 6 7.1 7.1.1 7.1.2 and Oreo (also with Andromeda rootless) are supported.
On 7.1-7.1.1 Substratum Legacy ROMs (including stock ones) you need to use an option called Frameworkless7.1RROcompatibility and it has white notifications.
Sadly it isn't possible to make them dark on that option.
Some manufacturers' ROM's aren't guaranteed to be compatible (the farer they are from AOSP or Google ROMs 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.

This is the icon pack that you can see in the first screenshot that I personally made to fit dark themes properly: https://play.google.com/store/apps/details?id=com.piereligio.darkpixelip


Themed elements:

-Pixel boot animation (on OMS only);
-Pixel and Nexus ringtones and UI sounds (on OMS only);
-14 wallpapers;
-14 accent colors (Pixel blue and Teal only on FrameworkLess option);

-Android N-ify XPosed module;
-AOSP Contacts;
-AOSP Dialer;
-AOSP Launcher;
-AOSP Messages;
-Calculator;
-Car Throttle dark mode;
-Documents/Download UI;
-DU Updater;
-ES File Explorer;
-Facebook Messenger;
-Framework-Res;
-Gboard;
-Google Assistant;
-Google Calendar;
-Google Chrome;
-Google Chrome Beta;
-Google Contacts;
-Google Dialer;
-Google Drive;
-Google Keep;
-Google Hangouts;
-Google Inbox;
-Google Launcher/Now;
-Google Mail;
-Google Messenger;
-Google Photos;
-Google Play Console;
-Google Play Music;
-Google Play Services;
-Google Play Store;
-Google Plus;
-Google Translate;
-Instagram;
-Kernel Adiutor's dark mode;
-LawnChair;
-N style notifications;
-Package Installer;
-Pixel Launcher;
-Root Explorer dark theme;
-Samsung Contacts;
-Samsung Keyboard;
-Samsung Phone;
-Settings;
-Solid Explorer dark theme;
-Spotify;
-Substratum app;
-SuperSU;
-SystemUI;
-Telegram (join the chat group of my themes and ask for the theme there to me);
-Twitter night mode;
-WhatsApp;
-XDA Labs;
-Youtube.

Thank you all for supporting me. :)

Layers Substratum Dark OMS RRO theme stock Marshmallow Android 6 6.0