Sky is an ambigender nature name that was first legitimized as the character of Sky Masterson in the 1950 musical Guys and Dolls, played in the film version by Marlon Brando. It's a name we appreciate for its clear, wide-open feel, less hippyish than others like Rainbow and Starlight, and makes an appealing middle name possibility.
Sky is the name of Sophie's fiance in Mamma Mia, and there are both Power Rangers and Transformers with the name. Elizabeth Berkley used it for her son.
It is also spelled Skye, as in the island in the Inner Hebrides of Scotland. Plus, it's a logical nickname for the variously spelled Schuylers and Skylers.