I remember when my PaperWhite glitched while I was attempting to jailbreak it and got stuck with the screen inverted. It was great! I can't believe I have to jailbreak a reader to get that simple functionality for better in-the-dark reading. (epilogue: I let it run out of battery and the inversion was gone upon boot)

You don't need to jailbreak it. Swipe from the top and press the big button labelled "dark mode".