Simply open the application and it will setup your turn your wallpaper black.
Features:
-Unlock googles hidden dark theme.
-Save you battery. OLED displays turn off black pixels.
-Reduce memory usage. Uses scalar vector graphics for the smallest possible image.
https://lifehacker.com/how-to-unlock-google-s-hidden-dark-theme-on-the-pixel-2-1819654177
*Lock screen may be overwritten by other applications.