Twitter rolled out an OLED-friendly dark mode called "Lights out" back in March, but it was only limited to iOS users. More than six months later, it's rolling out to Android users as well, albeit on the alpha version of the app.
The lights out mode is different than the current dark mode and turns the UI of the app to pitch black instead of dark blue. This does a better job at power consumption on devices that sport an OLED display.
If you are an alpha user, you will find the option to enable the lights out dark mode under the Settings and privacy > Display and sound > Dark mode appearance menu.
This OLED-friendly dark mode is available for users on alpha version 8.19.0-alpha.03 of Twitter for Android. But, it's activated through a server-side switch, so replacing the stable version with this one won't necessarily get you the new dark mode.
That said, some alpha users managed to activate lights out by clearing the app cache, so give it a try and let us know if it worked for you.