[Puppy]

I am wondering if there is any way to make it really dark as well as light in a certain area(so you could have a /light command for an interior).

[GGRoleplay]

You could try adding in a textdraw that is transparent but black so it seems like it's dark.

[Gammix]

GTA SA don't provide such features unless you use objects which have a static light range, color..
What you can do is edit textures of the walls to black or remove light sources from the room.