Below are the original pre-production sketches by William Ware Theiss. The skirt design for men, called a "skant", was a logical development, given the total equality of the sexes presumed to exist in the 24th century. It turned up on background extras during the first season. This was not a first, however, as Chief Engineer Scott wore a kilt as part of his dress uniform during the original series.

You wouldn't get me in one of those, even if it meant getting a "walk-on" credit

First season one-piece uniform

Later season's two-piece w/pullover

First season female uniform

Medical Smock worn by Dr. Polaski