The ground has lighting baked into it and does not seem to respond like the rest of the model with real time lighting. You could try blending in a black texture with the ground to darken it so it matches the surroundings. I created a black texture in an image editor and used the following code and it worked ok for a night scene:
Code:
blackTexture = viz.addTexture('black.jpg')
ground = city.getChild('ground')
ground.texture(blackTexture,unit=3)
ground.texblend(0.8,unit=3)
lanes = city.getChild('lanes')
lanes.texture(blackTexture,unit=3)
lanes.texblend(0.8,unit=3)
sidewalks = city.getChild('sidewalks')
sidewalks.texture(blackTexture,unit=3)
sidewalks.texblend(0.8,unit=3)
For the sky, you could set the background to different colors instead of using the sky texture.