Custom Dimension's Sky is always Overworld's Sky

I'm having the same problem with the sun, moon, and stars; I don't know how to remove them. I do know the solution to the sky color issue though. Make sure you're only using custom mod biomes for the dimension and that said biomes also have their sky color set to what you want. Hope that helps. :)

Update: I just found something in the code of one of the "dimensionname.json" files pertaining to the sky effects, and you're not gonna like it. The sky effects are determined by an "effects" variable, which can be set to "minecraft:overworld," "minecraft:the_nether," and "minecraft:the_end." Overworld gives the sky the sun, moon, clouds, and stars you're trying to get rid of, The Nether is essentially the same thing as enabling the fog effect on the dimension, and The End overrides the sky/fog color and just makes the sky look like the End dimension's sky. If you remove the line entirely, the game will simply treat it as if it's just set to the default Overworld, and you'll still get the sun, moon, etc. As far as I know, there's no way around this. If I find something else significant, I'll post another update here, but based on how this appears to work, I wouldn't count on it. I'm sorry, but at least now we know. Have a good day.