I recently installed Gunmod's Radiance lighting and the separate "World Lit by Fire" add-on for my medieval gameplay and my CAS is now dark. I am also using a custom CAS room. How can I fix this?

Have you checked if using the regular CAS room has the same effect with the lighting mods?

The radiance mod causes light to behave different, so if there's not enough lights in the CAS room or it's not placed correctly, or the wrong lights were used to build the room (affected more by one of the mods, so they give off less light), or no lights at all were used in the CAS room (relying on outdoor lights/light from windows and doors/doorways), this would probably explain the darkness.

Hmm, I'll take it out and see if that makes a difference.

thank you, that fixed it!

Then it's probably the lighting setup in the custom CAS that's the issue.