What is an atmosphere ?
Atmosphere is the general lighting of your level, let's say the weather, the daytime and so on. In GM Mode you can control this by the MOOD panel aswell.
You can define your working atmosphere as shown here:
https://docs.larian.game/Work_atmosphere_panelIf set to OFF it will use the defined atmosphere in the LevelSettings. For working (as long as lighting isn't important) i recommend the second choice and choose e.g. atmosphere "Work" or one that pleases you.
The third option is useful once you set different Atmospheres (by different atmospheretriggers) in your level, to choose from your different atmos.
You can also define your own custom atmosphere:
https://docs.larian.game/Atmosphere_panelSuggestion: Maybe this topic should be moved to GENERAL, to keep the guides subforum clean. Or we shall provide a guide to the problem here once it is solved.