Here is an excerpt from the new docs which will help you in obtaining a darker in door atmosphere:
GO light settings make it simple to control many of the light values including HDR lighting powers and fog colour. As there are many various parameters from the Directional light, HDR Exposure, Light power and even the environment power (Surface Reflection) this slider will quickly enable you to go from a day light situation to a dark indoor scene.
Why the fog colour? Well if you have a thick fog and you turn down the lighting you will be left with the current fog colour and if this is a light colour you may be left thinking that there is a light still in the scene. So when you turn down the light slider towards the “dark side”
the fog colour will be darkened for you whilst keeping the fog in the scene and giving the illusion that the fog it lit.