Hi ! Hollow Knight its basically one of my favourite games of all time, is for that I tried to study its artstyle, think there is three key elements here, (1) basically add a black outline to everything, and then (2) add a black gradient on the bottom of each sprite so it can blend both with other sprites, or with a black fill ( which would be your colliders ), and (3) finally try to make everything match with eachother, so use a limited pallete, and dont go too crazy on colours/saturation.