Starting out as a small modification specific to the Custom ROM ‘BlissPop’, Deep Darkness gained popularity to the point where XDA Recognized Themer _moelle decided to convert the modification into a full-blown Material Dark theme for the CyanogenMod Theme Engine.
With 500,000 to 1,000,000 installs on the Play Store, this theme can easily be tagged as one of the most popular themes out there.
Given the recent delay in development of the LineageOS Theme Engine, XDA Recognized Themer moelle decided to jump ship over to Substratum. He created a small private beta-testing group with around 60-70 members for Deep Darkness and I happen to be one of them.
Things to keep an eye out for will be the AOSP Keyboard which changes colors according to the accent, the minimalistic Quick Setting tiles, the custom animations and the beautiful boot animations.