Discussed previously and they said not possible.
The new feature coming next update will allow camera to switch to night/day on the ride. This is a good compromise.
On a related note, I've found that the 'temple gold block' pieces in Adventure pack can be changed to any colour. When changed to black they become the darkest pieces in Planet Coaster. It was a nice surprise to discover this.
Check out this box made of black "gold temple blocks". It's beautiful!
https://ozimg.s3.amazonaws.com/planet_coaster/black1.jpg
Inside the box I made a hole and put a black art shape in the middle. Yep... that art shape is black! See now how awesome the gold blocks are?
https://ozimg.s3.amazonaws.com/planet_coaster/black2.jpg
Of course, any scenery in there will reflect sunlight, so we are back to the solution offered in next update... switch camera to night time at certain points in your ride. This last image shows the combination of night time and black temple blocks. It's not pitch black, but it's very dark, and this should be enough for some good dark rides.
https://ozimg.s3.amazonaws.com/planet_coaster/black3.jpg