if it is hard try this,enda hapo kwa setting then display,turn night light on set times,peleka night light to more warm na iyo slider.view image for more clarity
I dont know what could be the reason why is not working,try restarting your laptop,it should work the night light might work,font size is not the issue
driver issues or your display cable is loose physically,first check which graphics you are using install its drivers,restart,if it fails its either the display cable is loose or your laptop is failing generally(kubeat)