We've known for a long time that dark space / night applications can extend battery life on smartphones with OLED displays. This is true of Android, and it's true for the iPhone. This is because individual pixels need to do less work in the dark areas of the screen and they use virtually no juice when they look real black. Like SlashGear Google re-played this on Android Dev Sum this week and shows a number of slides that compare several different colors. (The company used its original Pixel smartphone for this information.)
You can see that the white far and far away uses the most power. This led to Google recognizing that white features within its own apps and Android style guidelines are less than ideal. Its everywhere, and will not change with Google's renewed material design.
Fortunately, the company seems to recognize the value of dark space. YouTube and Android messages are already in use, and Google also introduces the feature to the phone app and tests it in the mobile version of Google Feed. Android can also be set to the dark theme from the shortcut drop-down menu and the app box, but Google has not yet advanced the system by adding the entire night space. (This is something Samsung plans to do with its new UI.)
As an example of the dark space to rescue the battery, just look at the huge difference below where Pixel is set to 100 percent brightness. The power saving of the dark space is obvious. I tend to follow the traditional look of these apps in most cases because the black text of my white text is a bit hard, but it's nice to see that Google recognizes the value of all white smoothing. Make it a choice in any sensible way.