you can create a cosmetic fog.
here's what you do
zoom out to 1/4 in the editor, and Screenshot your map
then open up photoshop (or gimp or something else that allows transparencies)
then, create a new layer and paintbucket fill black
make that layer semi-transparent, then create circular gradients from black at 0% transparent to black at 100% trans parent, over the lights
delete the screenshotted map, and export as a png
..........wow those instructions were convoluted...
if you don't understand them...let me know, and I'll try posting an example or something