Answer: Yes, but only on one with an OLED screen.
A recent test has shown that using dark mode on Apple’s newest lines of smartphones can get you a longer battery life.
The test, performed by PhoneBuff, pitted two iPhone XS Maxes against each other, with one set to light mode and the other to dark. The same tasks were performed on each phone using common platforms like Twitter, YouTube, Messages, etc. The battery on the device running light mode died first, at which point the dark mode phone still had 30 percent battery life remaining.
It should be noted, however, that this only applies to iPhones with OLED screens. Devices with LCD screens like the iPhone 11 or XR, although they were not tested, probably would not see the same benefit.