Are you referring to Dark Mode which is a setting that turns everything it can to dark ?
Or the setting that can help with eye strain called Eye Comfort Shield ?
The links explains how to use them but if they are not responding then first try a soft reboot by turning the phone Off and On.
If that does not help then try > Clear Cache Files from the phone's System Partition Section > https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/ You may need to insert your earphones or hook upto a powered On Laptop to access the Clear Partition Section. This may help with lag or stutters as this clears the phone of old/unused or broken cache files.There is also a Repair Apps option in the Menu System too.
Ultimately back up and factory reset as a very last resort.
If I can be of any further help then please don't hesitate. Take care. 😎
Daily Driver > Samsung Galaxy s²² Ultra 256Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "