How do I

Is it possible to darken a layer without darkening layers behind it?

I'm attempting to do lighting similar to Terraria, the ground, trees, player, items ect got darkened at night, but the night sky was still bright and clearly visible.

Yes, use system> "set layer opacity" ...

Actually, I'd select the Layer you want to alter, then on the right hand side look for Effects, Add/Edit effects then add the Brightness effect and alter it's brightness to say 25-50 when night and 100 when day

— Awesome thank you