Google has compared using Dark mode on applications to show the difference between standard mode and Dark mode and its end result is impressive in Pixel devices. Google claims that using Dark Mode in AMOLED Devices and most specifically in Google Pixel devices, will relatively improve battery performance.
Google did not simply show the report numbers, they showcased the battery life on devices with the impact of different colors. Black tops the chart and followed by Red, Green, Blue and the extreme battery draining color- White on the last. Youtube has also got Dark Mode option and also the messaging app.
Android Pie has options to customize the Dark theme mode which darkens Google Feed and Quick settings which massively reduces battery draining in Pixel devices and other AMOLED smartphones.