Take a look at NickelMenu. You can still do the Invert mode but it does require a reboot.
Add the following item to your NickelMenu config file:
Code:
menu_item :xxxxx : Invert & Reboot : nickel_setting :toggle: invert
chain_success : power : reboot
Replace the xxxxx with where you want to see this item (main, library, reader, browser, selection). Selecting this item will toggle the mode and restart your ereader.
Invert mode was never an official option and before NickelMenu, it involved installing a small file and then playing with how to get into and out of Invert mode.
Personally, I prefer Dark mode to Invert mode since Invert not only inverts text, it inverts images.