I imagine there could be veins of crystal through the Spire rock, carrying light through the Spire. Essentially, built in lighting strips.
But there's no textual evidence for it (yet).
It would have to be pretty efficient, but I suppose it could work. Still, even this would have weird secondary effects. As an example, diffuse lighting through the habble means very few true shadows to hide in. Everything would like the interior of a shopping mall -- just soft shadows everywhere. Also, you've got habble Landing. With extra roof they built in the middle, the lower level wouldn't have access to this lighting. It would make for a very dark place unless you've got artificial light.
As second issue could be radiation. Living indoors would mean that nobody ever got a tan, giving everyone (except for sailors) a very pale complexion.