Dark mode and invert mode are not the same thing. The former only inverts black-and-white in actual books (while leaving pictures correct) but does not invert the user interface or home screen; the latter inverts everything including pictures. This Nickel menu code when let you control both modes independently (note that switching invert mode on/off requires a rebook, with this code will call for automatically).
Code:
# -----------------------------------------------------------------------
menu_item :main :Dark Mode :nickel_setting :toggle :dark_mode
menu_item :main :Invert & Reboot :nickel_setting :toggle: invert
chain_success :power :reboot
# -----------------------------------------------------------------------
menu_item :reader :Dark Mode :nickel_setting :toggle :dark_mode
menu_item :reader :Invert & Reboot :nickel_setting :toggle: invert
chain_success :power :reboot
# -----------------------------------------------------------------------
menu_item :library :Dark Mode :nickel_setting :toggle :dark_mode
menu_item :library :Invert & Reboot :nickel_setting :toggle: invert
chain_success :power :reboot
# --------------------------------------------------------------------------