awe thanks~
Here's a simple breakdown~
It's mainly reliant on the lineart, I reccomend maybe copying a manga panel you like to get a feeling on where to draw the lines thicker and thinner. I did this with a manga called Arago a long time ago.
For this game specifically I tried not giving the characters any highlights in their eyes + gave them eyebags to make it feel dark. I also used a warm color palette to make the game feel a bit older.