Seeking name of an older (possibly 80s?) fantasy book. Was a fairly thick paperback that I think was part of a series, but my library only had the first one.
Details: the main character is a teen/young adult female, possibly with red hair. Has a male friend with a limp/damaged legs?
Vague plot I remember is there is no visible sunlight due to pollution, so it is always cold and dark. Labourers/slaves are forced to mine coal (that they call by another name which I dont remember) to keep richer people warm.
Female lead is one of the labourers. She somehow discovers that magic can be used to keep warm instead of burning coal, and teaches this skill to others. I think there is a revolt at some point?
A sun symbol is a reccurring theme, but it is mistaken for a lion symbol at the start, because no one has seen the Sun for generations.
At the end of the book, the Sun is visible again, due to reduced coal burning.