Open Roblox Studio
In Explorer, select Lighting
Set TimeOfDay to 00:00:00
Or set ClockTime to 0
Set GeographicLatitude to 0 if needed
Disable or remove any scripts that change Lighting time
Set Brightness to a low value if you want darker night
Set Ambient to a dark color if you want a night look
Set OutdoorAmbient to a dark color if you want a night look
Set GlobalShadows to true if desired
Lock the Lighting properties in your game logic by not updating them during play