How to solve my mi 8 black screen out issue?

It is because you downloaded black theme for your mi 8. Would suggested to factory reset the phone.

Maybe 1) the display is damaged 2) the display functions normally, but cannot receive information from the processor, some of the connecting parts must have problems.

As you have mentioned, it gradually become back, so it seems to be 2).

The phone functions correctly except for the display. I recommended sending it back for repairing. It will be very risky as you cannot determine which connector(s) are faulty, and it did not seem to be a software problem.

Hi. My phone is water resistant. Put underwater for 5mins and the screen suddenly blackout (charging and power on is working properly). 2hrs later, I disassemble everything and clean with wet and dry cloth and assemble back. Screen still black, went to phone shop and they recommend to change motherboard. I change but the black screen still happen except everything still operate normally