Hello there,
On the top left, one of your options is "Map Aspect". Click on that and you will get a map characteristics menu. Towards the bottom is "Fog". Any intensity above 0% will make the "fog" visible. However, it seems to be map wide. It is still very useful when you tint it in dark colors to make a "night" map, as it will light up with "light sources" quite nicely.
Overall, a very good map maker tool.
Cheers!