Current questions and answers regarding the map editor.

Froska, what editor should I use to edit the illumination file? Notepad?

Map editor ;)
You make a copy of the illumination, change it as you see fit, and export it to the map. I hope it's not too difficult to understand?
Here's where to find the settings to modify:
Map properties on the left-hand tree panel -> GroundAmbientLights -> 0 -> ... -> and you'll see the menu you need. Right-click and copy the illumination file. Then paste it anywhere and experiment without fear of ruining the map.