Import a sky dome prop
After you set the city elements, you could import a sky dome such as the Colors of Sky prop.
Hint:
How to make a sky animation? You could use time line to edit rotate duration of the sky dome 1. Click the Sky Dome to use in the scene, and open the Timeline.
2. Open the Transform channel, and click the 100 frame.
3. Rotate the Sky Dome, and Timeline will set keys automatically.
4. Play to see the result. (you can use the same method to add light animation).