(written from a Production point of view)
William "Bill" Ware Theiss (20 November 1931 – 15 December 1992; age 61) worked as Costume Designer for the entire run of Star Trek: The Original Series and for the first season of Star Trek: The Next Generation. Theiss received credit as Costume Creator and Executive Consultant on the first season, winning an Emmy Award for the episode "The Big Goodbye" in 1988. In the following seasons he was credited as "Original Starfleet Uniforms/ Starfleet Uniforms Created By", in the process earning a second Emmy Award nomination for "Elementary, Dear Data".
Theiss' distinctive clothing style for the "The Veldt", a mid-1960s one-act play from "The World of Ray Bradbury", caught the attention of DC Fontana, and introduced Theiss to Gene Roddenberry. Getting along on a personal level, Roddenberry, who was already interviewing candidates for the position, signed Theiss on as costume designer for his Star Trek pilot, and eventually the upcoming television show.  While Theiss has designed recognizable utilitarian clothing for the show, the most notable ones the iconic Starfleet uniforms, it were his more explicit female garment designs, especially those for "guest" characters, that drew the most attention, as well as some controversy, among others in feminist circles. Even his female Starfleet uniforms were not beyond reproach, in regard to the short skirt length. Decades later that would be playfully hinted at, when writers would have Jadzia Dax dry-wittingly remark, "...And women wore less..." in Star Trek: Deep Space Nine's fiftht season episode "Trials and Tribble-ations", as she dons a Theiss-style uniform. (scene 20) His female garment style can be summed up in the "Theiss Theory of Titillation" (self coined and first mentioned in The Making of Star Trek, p. 360), which stated "the degree to which a costume is considered sexy is directly dependent upon how accident-prone it appears to be." Nevertheless, over the next decades, his garments would attain near-legendary status, and a number of them, including some of the more titillating ones, have been for the first and only time, before being auctioned off, on public dispaly at the 1992 Star Trek Smithsonian Exhibit. 
Once the outlines for the new television show, Star Trek: The Next Generation, were in place, Theiss was invited back by Roddenberry to join the new show in early 1987. He started working from what he had done on the original show, and discounted what has been seen in the Star Trek films up to then. While the majority of his designs were utilitarian, a few harkened back to his titillating ones of the Original Series, most notably, the crisscrossing harem girl outfit worn by Tasha Yar (Denise Crosby) in "The Naked Now", and the chest baring costume worn by Commander William T. Riker (Jonathan Frakes) in "Angel One". Theiss recalled at the time, "Denise has a terrific body and was comfortable exposing it to a degree. She had no problem with her costume." Frakes, however, was more selfconsciuos, "But that was fine, because as Riker, he was supposed to be uncomfortable with the outfit, and Jonathan is enough of a professional that he used his own uneasiness to his character's advantage. That episode was really tough to design for.", Theiss continued. 
Asked which of his many creations, he counted among his favorites, Theiss has stated, "The gown for Leslie Parrish in "Who Mourns for Adonais?" seems to get a lot of attention. [at the time Stephen Whitfield reported, that while Parrish herself was completely comfortable and had no qualms about wearing and moving in it, it were "the crew and set visitors who had all the qualms" (The Making of Star Trek, p. 361)] Other than that, I don't know. Michelle Phillips' outfit in "We'll Always Have Paris" is quite sensational; and Denise Crosby's punk-of-the-future costume from the sewer scene in "Where No One Has Gone Before" is good. The problem is that a lot of my work is seen on screen for only two to three seconds, and even then it might be in a bad light or angle. Like Commander K'Nera's uniform in "Heart of Glory". I was very proud of that."  Incidentally, Theiss was not wrong in his assessment of Parrish's gown, as it brought in ten times the high estimate, when it was sold in the below mentioned auction of 1993.
Theiss was perceived as a demanding and hard driving man by his staff of dressers and costumers, impressed upon by his adagios such as, "Stop when all work is done–and not before", and "Better rude than late". Colleague and customer Andrea Weaver noted in this regard, "Bill Theiss was a very creative designer. His designs for Star Trek were original rather than distilled from other sources, or redefinitions of previous work. This is what I appreciated about Bill Theiss. I thought he was a truly unique and rare costume creator. Others may have agreed but were more influenced by Bill's personal eccentricities and rudeness..." (Star Trek: The Original Series Sketchbook, p. 119)
An attendee of the "Art Center College of Design" in Pasadena, California (which also counts Andrew Probert and Mark Stetson among their alumni, and like Probert following after his service in the United States Navy), Theiss' first job was as Cary Grant's personal assistant, whose ex-wife, actress Dyan Cannon, Theiss has cited as having considerable influence on his career. (Star Trek: The Original Series Sketchbook, p. 116) His first contribution to films was the 1960 movie Spartacus, albeit uncredited. Theiss continued to work in theater, film and television for more than thirty years. His work included among others the 1971 films Harold and Maude (featuring Ellen Geer and photographed by John A. Alonzo), as well as Pretty Maids All in a Row (produced by Roddenberry). During his career Theiss has earned three "Academy Award for Costume Design" nominations for Bound for Glory (1976), Butch and Sundance: The Early Days (1979), and Heart Like a Wheel (1983). A private man, Theiss was never one for interviews, the 1988 unpublished interview, on the occasion of his Emmy Award win (see link below), the only one known to date.
Theiss died in 1992, a victim of AIDS, at the age of 61. Theiss apparently has retained ownership over most of his creations, as a large number of his Original Series creations were sold off as his estate in the 1993 The William Ware Theiss Estate Auction, the proceeds of which he willed, poignantly appropriate, to "Project Angel Food", a LA-based non-profit agency that served hundreds of meals on a daily basis to those challenged by HIV-AIDS, cancer and other diseases.
- 1988 Emmy Award win as "Costume Designer" in the category "Outstanding Costume Design for a Series" for TNG: "The Big Goodbye", sole winner
- 1989 Emmy Award nomination as "Starfleet uniforms creator" in the category "Outstanding Costume Design for a Series" for TNG: "Elementary, Dear Data", shared with Durinda Wood, costume designer