I am thinking most light sources are going to arcane in the form of glowing runes, and "sun beads" strung along the ceilings/between the streets. Or be natural and take the form of oil lanterns. I want to keep with my colour themes, so all the lights would be white, yellow or orange/red in colour.
I know this might sound repeated as its used in the Underdark, but glowing moss could work in your favour. If the wall was never designed to be lived in, having a natural light source conveniently available for the new residence could be taken as a sign of why they moved in.