What is the effect of looking at a girl—even without lust—on the face’s light and darkness, and what is the effect of lowering the gaze in that regard?

A man looking at a non-mahram woman is forbidden, even if without lust. Sins and transgressions have numerous effects and punishments in this world and the Hereafter, among them the darkness of the heart mentioned by Ibn al-Qayyim. It causes confusion in the heart and manifests as blackness on the face. Ibn Abbas said: "Indeed, a good deed brings light to the face, a radiance to the heart, abundance in provision, strength in the body, and love in the hearts of creation. And indeed, an evil deed brings blackness to the face, darkness in the grave and the heart, weakness in the body, deficiency in provision, and hatred in the hearts of creation." This darkness or light are characteristics that appear on faces and are not related to skin color.