Sun and Night Types

---"But Dark"

No. The Dark type is not about night-time or darkness as an element.
The Dark type (which in japanese is essentially called "Evil type") is all about dirty moves, sneaky tactics, shady tricks, or just unfair attacks like biting.
Even when you look at the description of that one Dark move that seems like it is an elemental darkness move, Dark Pulse, you find out it's about giving off an aura of horrible thoughts, not attacking with darkness.

---Light and Darkness

and that's why I'm always arguing against "Light" type ideas. Because most who suggest it, do it based on the assumption that the english name for Dark type is to be taken literally and hence want to give it a proper counterpart.

But if I add an actual type about elemental darkness, a type about light suddenly makes sense as well.
Hence Night and Sun types.

(unfortunately it doesn't go well along with my other type retcons, due to quite some overlap, in particular with Cosmic)

---Lycanroc Dusk/Twilight?

It's not a night-focused form. And twilight is still some sunlight, so the solution is simply have it be Rock/Sun as well, or just pure Rock :'D