It depends on your software version. If you are running on anything > 5.14.2 then you're out of luck (as of now).
* for 5.12.2.2 - 5.14.2: WatchThis JB
https://www.mobileread.com/forums/sh...d.php?t=346037
* for 5.10.3 - 5.13.3: KindleBreak JB (easier)
https://www.mobileread.com/forums/sh...35#post4105735
After you jailbroken and installed KUAL, then try this Fake Night Mode KUAL extension
https://www.mobileread.com/forums/sh...d.php?t=338483