City lights could have been added before in 0.9.8.0, but would remove normal volcanoes, since city lights are technically volcanoes. This is the code I used for making city lights on my city planet in .980:

DiffTileSize 258
DiffTileBorder 1
GlowTileSize 258
GlowTileBorder 1
GlowMode "Night"
GlowColor (0.70, 0.67, 0.40, 0.50)
GlowBright 3
Velocity 0.0
volcanoMagn 0.174
volcanoFreq 1768
volcanoDensity 256
volcanoOctaves 16
volcanoActivity 300
volcanoFlows 0.90
volcanoRadius 0.90
volcanoTemp 1e+007
lavaCoverTidal 0
lavaCoverSun 0
lavaCoverYoung 0

If anyone knows how to readd city lights without messing with the textures in 0.9.9.0, then please put the code down in the comments.

EDIT: City lights weren't officially added, but rather, it was accidental, and the only planet with city lights is Earth. The city lights themselves don't have values except for messing with the volcano code. There aren't any procedural "cities"/city lights in the game.