Xposed Module Brings Reading Mode on MIUI 8 Devices with Custom Kernels
If you are using a custom kernel on a device with MIUI 8, then the stock Reading Mode feature may be unavailable. This feature tints the screen to filter blue light from the display so you can read easier at night. If you would like to use this feature on your MIUI 8 device, then check out this Xposed Module called PM Back by XDA Senior Member S0bes. It hooks into the KCAL advanced color control kernel interface and forces the MIUI 8 Reading Mode to operate based on KCAL. This means you can use the feature again without changing the display calibration mode through a kernel tweaking application.
MIUI 8 Reading Mode with a Custom Kernel
Grab the PM Back Xposed Module for MIUI 8