Snowbun, some of the people you see covering their legs with black tights all the time, for instance black tights at a wedding with slingbacks, think they are being fashionable. Perhaps erroneously. This is not to be confused with the people who wear them as a chumra, in which case it’s not Succos to Pesach- (why would the season make a difference as far as tznius is concerned?) it’s all year.
As for the exact halachic requirements, I would like to leave that to the more learned posters who can quote the relevant sources and offer proper references for what they say.
Feel free to consult me on matters of fashion, though.