for Dark Corner. this usually happens when trying to connect LCD cables. but there's a misalignment. misalignment means not all pins got fitted in. so for some backlight parts it would be ok to turn on. but some might not.
in order to solve this, you need to remove all the cables and also must remove the battery clip to let it power off "completely". ( capacitor issue if you know some electronics)
and then connect them again. then it should work.
please check below video
