Dark Light is a game mode in which you need to complete a vanilla OneShot with some conditions:
1. 45 minutes to complete
2. Fast travel is disabled
3. You cannot close the window (Nico will die, dreams do not count)
4. If you do not complete the timer, the game will close and Nico will die
The rules window before starting:
What the timer looks like:

We go to the search bar %appdata% ---> OSFM
Next, we need to delete two files save_backups and p-settings.dat :
P.S. After entering the game, the p-settings.dat file may be restored, if this happens, then exit the game and delete the file, do this before the black screen and dialogue appear
After the dialogue appears, select "Dwell on the past (replay vanilla)" :
After that, the fourth option should have the "Dark Light" button :

I wish you all the best of luck with your playthroughs!