Did you manage to find Settings => Reading Settings => Dark Mode (on a Sage, it's on the second page of the Reading settings). If not, what firmware version are you using?
You can also use NickelMenu to allow fewer taps to access this and Inverted mode though Inverted mode does require a restart.
You will need to add the following lines to NM's 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
#