Some people also wind up talking to their jewelry, but that's mainly in more isolated cases.
A thing that's easy to forget: Light might be the main source of energy on the surface of Earth, but it's not the only source of energy on Earth, and definitely not the only one on Yliakum's world. Other potential sources:
Geothermal (if I remember right, between two to five miles down on Earth is hot enough to boil water!)
Magma (technically it's part of geothermal, but magma specifically can provide light too)
Radiation (could be harnessed directly I'm sure, but it can also heat stone and water, or cause certain substances to emit light)
Hydro (underground rivers, waterfalls, tides, currents, etc)
Magic (but I don't know enough about PS's specific version of magic to give good examples)
Chemical (for example, if some form of plant or creature could consume coal or oil, or if Kran-like creatures could metabolize more rock-like substances)
Wind (if there's a very long tunnel or large area, heat differences could cause this)
Fusion/Fission (Not natural and seemingly unlikely, but who knows what inhabits the depths? As far as I'm aware, even Talad and Laanx haven't been to the other side of the planet. For all we know, there might be a technologically advanced race of mole men out there with fusion power plants).
There are other sources too, but I don't know enough physics to know much about them (for example, if the planet has a magnetic field, chances are that field is going to be constantly experiencing slight changes. How much energy is available and how it would be harnessed, I know not. And then there's tectonic movements, tidal effects from the sun, moon(s), and other planets, pressure differences, planetary rotation, etc.)
Sure, some of these might seem a little unlikely, but think about it: Plants can absorb sunlight and convert it into energy. Human skin can use sunlight to help produce vitamin-D. If you stop to think about it, that's no less "out there" than anything I mentioned.