At the bottom of the home screen, tap on More => Settings => Reading settings and on page 2 (might be page 1 on an Elipsa), tap the On box for Dark Mode.
Alternatively and the way I do it is to install NickelMenu and add the following to the configuration file:
Code:
#
menu_item : reader : Dark Mode : nickel_setting : toggle : dark_mode
menu_item : reader : Invert & Reboot : nickel_setting : toggle : invert
chain_success : power : reboot
#
Actually, this included Dark Mode and Inverted Mode (Inverted requires a reboot after enabling it).
With that enabled, I tap on the screen, tap on the ... menu, tap on Dark Mode and I'm in business.
Dark Mode only applies while reading a book and does not invert images while Inverted mode applies everywhere and to everything.