But the Trickery domain includes exactly one feature or spell related to darkness in any way.
A large part of that is that Darkness is a terribly limited concept to build a sub-class around. But even then, I think Trickster is better than you suggest. All of the Stealth abilities I would say directly key into darkness as a concept (pass without trace, blessing of the trickster, cloak of shadows); the two travel spells (blink and dimension door) are a reasonable fit as well. The other stuff is less thematic, but it is not glaringly so.
The light domain is already fine for a pure arsonist. Huge selection of spells for setting things on fire (Burning Hands, Flaming Sphere, Scorching Ray, Fireball, Wall of Fire, Flame Strike). And some stuff about using light to blind people and doing more damage. I do not think it is really 'good' themed at all.
Cleric of Clean Still Air? I am not really sure that stands as a sensible concept tbh, you certainly couldn't really do it in prior editions. It is not something I would allow in my game, it is just too specific.
I do agree that Tempest does not represent Fire very well or Earth at all.