Dark
1.

2. Eye Scream another obvious one a demon with eye ball minions.
3. Fisticuffs is a brawler who probably enjoys underground battles in darkness like dark groups who sport private dark battles underground.
4. Tae Kwon Crow is one a lot of people question and here is my answer. He is a shadow ninja trained in the dark arts of martial arts. While he doesn't sport a stealthy move like Nightshade, it is pretty obvious he is an evil rogue ninja who had probably played dirty and uses his martial arts skills for evil.
Light
1. Luminous: again no need to explain, but he's a being of pure light.
2. Lob Goblin always talks about electricity which is mainly classified as light. That is why he is all bout light and not tech.
3. Eye Five shoots light from his eyes and energizes himself so he can move faster. Also plays with irony that an eye monster would be about light when it could easily blind him.
4. Blaster Tron is a tricky one to describe, but he is a robot purely based on light attacks. Yes he's a robot and could have just been a tech villain, but remember he uses infrared light beams to scan and shoot enemies and seems to be powered solely by light energy so that is why he is in the light element.
Whether or not you agree with my opinions, I just wanted to help clear the confusion and hate it when something is not clearly explained. I want to at least have a better idea as to why they chose these elements for these villains.