Ever since OLED display were introduced in the smartphone and tablet industry, we knew that dark themed apps will help improve the battery life of said devices, because a pixel turned off (not consuming energy from the battery) is always black (that deep black OLED technology can achieve compared to LCD panels washed out colours).
But for some reason almost all Android developers are creating apps with their main theme in white colour. And, in many cases, they do not offer the option to switch to a dark theme within their apps.
In a keynote at Android Dev Summit 2018 in Mountain View, California, Google confirmed the positive impact of the Dark mode on the battery life of the gadgets. The speech was intended to educate developers on how to improve battery life by introducing dark mode in their apps.
For a more detailed explanation check Slashgear’s article.