Hey maxi!
Lightweight scenery basically means that your track shouldn't look empty, but the scenery shouldn't distract the driver and you shouldn't fill every block with something. You could check the new STC maps (just search STC in 'Find Tracks') for example, they'll give you an idea of what the scenery should be like. There isn't any fixed copper limit that you can't go over, you should just try to make the map nice-looking with a few scenery elements
All moods are allowed. You can't change the fog color or anything in the intro/ingame/outro MT, but feel free to edit podium cameras!