I'm a bit annoyed to this day that they didn't re-do all the planet skies to look like more than just dinky colored backdrops. Putting in clouds, the occasional bird or bird-like creature, a sun, a moon or three, that would've gone a long way to help the show look better, as well as taking all the upper displays on the bridge and actually animating them so they don't look like little more than portraits.
I'm pretty sure they did add some "sky effects" to several episodes. I know we see two moons in the sky in Bread and Circuses
Replacing the upper bridge monitors would've broken the meager budget CBS gave them for the project. I imagine TNG will receive the same meager budget.