A young woman searched the lobby of a theater showing the film ''Out of Africa'' one recent night, turned to her companion and said, ''If they had a concession stand selling the clothes in this movie, I'd buy a whole new wardrobe.''

Lately, at the Beverly Hills branch of Banana Republic, a retail chain specializing in safari and travel clothing, customers have been asking to be outfitted in ''Out of Africa'' styles.

Milena Canonero, the designer who has just earned an Academy Award nomination in costume design for the rugged but sensual safari styles of ''Out of Africa,'' has heard such talk before.

In 1981, she dressed the athletes in ''Chariots of Fire'' and her Academy Award-winning 1920's designs inspired a fashion trend. Ever since her first movie, in fact, she's been turning film into fashion. The futuristic villain-heroes of the 1971 film ''A Clockwork Orange'' inspired cult styles among London's youth.

'I'd Hire Her'

''She is one of the best costume designers in the business,'' according to Ralph Lauren, who says he has been influenced by Miss Canonero's work. ''She's quite fabulous. If I were doing a movie, I'd hire her.''

Miss Canonero declines to give her age, and that refusal to bow to time is consistent with her fashion philosophy. Though the strikingly attractive brown-haired designer wears contemporary sportswear, her professional forte has been period costumes, both of the past and the near future. Her conversation in a recent interview cut across time too. ''I'm in the middle of a whirlpool,'' she said. ''You don't know where is the beginning and where is the end. I hate time. There is never enough time to do something properly. There is never enough time to live all the lives you want to live.''

Miss Canonero ''went to the movies all the time'' in her native Italy, she said. She studied fashion, period design and art before moving to England in the late 1960's. There, she designed for friends' boutiques, ''which one day were there and the next day they were not there,'' she said. Her main interest was movies, however. ''Cinema kept calling,'' she said. ''One way for me to get into the movies was to make costumes.'' Working on commercials in London, she met directors, including Hugh Hudson, who would go on to make ''Chariots of Fire,'' and a cameraman who was Stanley Kubrick's director of photography. Miss Canonero was invited to watch Mr. Kubrick shoot parts of his film ''2001'' and he asked her to work on ''A Clockwork Orange.''

''He gave me a break,'' Miss Canonero said. ''I learned a lot working with him. Plus he takes time to do his films. He has total autonomy. You have one boss, Stanley Kubrick.'' Inspired by Street Styles

The costumes in ''A Clockwork Orange'' were inspired by then-current London street styles. Malcolm McDowell, the film's star, and his gang of stylized ruffians wore black boots known in England's youth cults as ''bovver'' -meaning bother - boots. After ''A Clockwork Orange,'' a new cult, Bovver Boys, reviled for their tendency to cause ''a spot of bovver,'' as in ''Clockwork Orange,'' sprang up in London. Rock musicians began sporting the film's white pants with suspenders, black bowlers and bovver boots. Young women in the film wore color-streaked hair, a style that would resurface in punk rock and new wave circles.

Miss Canonero won her first Academy Award in 1975 as the costume designer on her next film, Mr. Kubrick's late-18th-century costume epic ''Barry Lyndon.'' Then came an offer to design costumes for a future period, the world of George Lucas's ''Star Wars.'' She turned it down. ''I was tired, exhausted,'' she said, ''so I lost that wonderful opportunity. That's the only thing I regret to not have done. I've made fairly good decisions up to now.'' Though some of her films ''disappeared,'' she said, smiling, others gained wide audiences. She worked on ''Midnight Express,'' Mr. Kubrick's ''Shining'' and Mr. Hudson's ''Chariots of Fire,'' which won Miss Canonero a second Oscar for costume design, in 1981. That film helped to inspire a fashion trend, along with television's ''Brideshead Revisited.''

Men's-wear designers, including Ralph Lauren and Jeffrey Banks, introduced clothes that echoed the film's nostalgic British tailoring. Tweeds, draped jackets, vests, argyle sweaters and argyle socks quickly became popular. ''For me, as a designer,'' Miss Canonero said, ''it was a wonderful lucky stroke. We're working on a movie, we're not thinking of fashion headlines. Sometimes it clicks with fashion.'' When that happens, ''you feel great,'' she said.

Following ''Chariots,'' Miss Canonero was asked to design clothes for Norman Hilton, a men's-wear manufacturer. She didn't repeat her film designs, though. ''I wanted to do American clothing,'' she said. That collection won her a special Coty Award.

Miss Canonero returned to period costume design with Francis Ford Coppolla's ''Cotton Club.'' The video version, she has heard, will incorporate footage that was cut from the commercial film. ''On such a big picture so much gets done and it's not there,'' she said with a sigh. ''It's frustrating.''

Her work on ''Out of Africa'' began last June. After reading the script, she studied the garb of tribes native to Kenya, as well as that of the early-20th-century white settlers. She went to libraries, embassies and museums in Nairobi and London. She also talked to the contemporaries of the author, Isak Dinesen. ''You find this incredible guy who lives all alone in Yorkshire who's got the biggest collection of Somali references,'' she said. ''You photograph everything. No matter how well you know a certain period, you still go into it.''

''Once I've done my research,'' she said, ''I go into what each character should wear. For a period movie where period clothes exist, you try to combine real things with things you design and make. The real thing, you can never make it as good as that.''

What's next for Milena Canonero? ''I'm interested in evolving,'' she said. ''I'm interested in making money. You don't make money by designing costumes for movies, unless you churn out work one after the other. I don't want to become a machine. The fashion business offers opportunities. It's interesting to explore them.'' Then she smiled and paraphrased one of her ex-employer's films: ''If somebody makes an offer I cannot refuse, I'd be very interested, yes.''

photos of Milena Canonero and some of her designs (NYT/Bill Cunningham)