When The Theatre Complex opens here on New Year's Eve, it will herald not only 1980, but the 21st century, too.

The Theatre Complex, which is unlike any other theater building, is part of the spectacular Denver Center for the Performing Arts. Its central performance space consists of two seating areas so arranged that one vast stage can be used for a single production; two smaller ones for different plays in a single production; or two smaller ones for different plays before separated, simultaneous audiences of 750 and 600 seats each. Set in diverse tiers, seats can be raised or lowered to accommodate varied types of staging and lighting, unprecedented in previous theaters.

The theater's setting is as spectacular as its staging potential, one of five buildings linked by a cruciform galleria, its glass roof 80 feet above the pavement, where cafe tables will have strong appeal for lunch-house stroller. Looking west are the snow-topped Rocky Mountains. On a clear day, one can see Pike's Peak, 75 miles away. The whole is an ingenious update of those vibrant gallerias of Milan and Naples.

At the south base of the cruciform is the Auditorium Theater, built in 1907 as a traditional picture stage. Adjacent is a more recent sports arena, envisioned for furture television studios. Across from them is a 1,600-car parking garage, the size of both buildings combined. Opposite The Theatre Comlex, at the top right of the cross, is Boettcher Concert Hall, a circular hall of impeccable acoustics opened a year ago, its "surround" design resembling the Berliner Philharmonia.

While the east-west glassroofing is completed, the north-south axis enclosure is several years away. But with the Dec. 31 opening of the new building, the project basically will be complete.

Created not by a single architect, but by several firms, with the Mark Taper Forum's Gordon Davidson as consultant for the theater, the center is in the heart of Denver's lustily revived downtown area, not far from the confluence of the Platte River and Cherry Creek, where Denver was born in 1858. Relocation of Speer Boulevard will give a park-like setting to the west end of the complex and include, eventually, an open-air amphitheater for summer use.

There is nothing like this performing-arts center anywhere. Considering inflation, it's lucky it was started when it was - in the early 70s. Former Arts Endowment chairman Nancy Hanks recently said that Denver's center "will have to be the last" of the big performing arts complexes.

How this melange of art and architecture, culture and cash, independence and interdependence, past and future, came about is as complex - yet as direct - as the history of this scrappy "mile-high city," where the plains meet the mountains.

In an imaginative flash, a North Carolina Lawyer got on the phone, outlined his vision and before he ung up, had the Denver mayor's vital support.

"It took me only two minutes, standing on the site, to sketch it on the back of an envelope," says Donald R. Seawell, trim and golf-addicted at 66, instantly comes across as the elegant leading man of those Philp Barry-Noel Coward comedies that set styles for young men in the 1930s. Son, grandson and great-grandson of North Carolina Supreme Court Justices, young Seawell turned naturally to law.

Hearing him debate (after Winston Churchill had jolted the debaters by switching their topic), Joseph P. Kennedy informed young Seawell that if ever he sought a legal slot in Washington, he'd pave the way at the Securities and Exchange Commission. Passing the North Carolina bar, Seawell sought the job. He lived stylishly in a Massachusetts Ave. mansion that its half-dozen residents had dubbed "The Monastary."

World War II packaged Seawell's future. He met and married a young actress. Eugenia Rawls, who played Tallulah Bankhead's spirited daughter in "The Little Foxes." The Army assigned him to counter-intelligence and he wound up in the initial team of 35 men in General Eisenhower's London command, which introduced him to all the great leaders of the day. On his foundation of corporate law, he would build a career-long international clientele.

In time, his clients included such biggies as the Lunts, the Oliviers, Noel Coward, John C. Wilson and, first among equals, Tallulah herself, who insisted on becoming godmother to both the Seawell children, daughter Brook and son Brockman, who grew up to be performers utterly delighted with their activist parents.

Army duty opened doors in both Washington, where he created and chairs the Civilian/Military Institute of the Pentagon and in England, where he created and chairs the Civilianland, where his club is Bucks, his Ascot seat is in the Royal Enclosure, and his clients include Virginia McKenna, who next week opens in London opposite Yul Brynner in "The King and I." The peripatetic Seawells will be there.

As a Broadway producer, Seawell clicked with "A Thurber Carnival." Besides becoming president of The Players, he's a member of a half-dozen theater boards and also beame chairman of ANTA, the American National Theater and Academy.

Which is what led Seawell to Denver, for through ANTA he came to know actress, coproducer, client and friend, Helen Bonfils. Open-hearted Bonfils never became a Broadway star, but in Colorado being a Bonfils amounted to being royalty - awasome to some, despised by others.

In 1895, Frederick Gilmer Bonfils and Harry Heye Tammen created The Denver Post. "Tam" and "Bon" got their circulation going by selling their subscribers coal at 50 cents a ton cheaper than the coal trust demanded, a procursor of circulation gimmicks that would make the paper the largest and richest in what it would christen "The Rocky Mountain Empire."

The lawsuits in which Fred Bonfils involved himself were finger exercises for the wills he and his widow created, leading to a split between their two daughters. When May, the elder, made a marriage her father disapproved of, tilted his paper's future control to Helen, who remained in the Bonfils mansion and didn't marry until she was 43-after Fred's 1933 death. With the paper worth millions, sister May in 1960, after years of restrained fuming, formed an alliance with the Newhouse chain to wrest control from sister Helen. That provoked a 12-year legal battle, which wound up in Helen's favor, though she died a few months before her-and Seawell's-victory.

But Helen, trusting Seawell, had decided what would happen to the paper's future profits. Just under 10 percent would go into the Post Employes Stock Trust, a decidedly progressive step for the paper. About 91 percent from the two stock-holding foundations, the F.G.Bonfils and the Helen G. Bonfils, would go to Helen's projected dream, the Denver Center for the Performing Arts. Active in all phases of Denver arts and creator of a handsome building for strictly community players, she provided that all her allied arts interests would come under the center's umbrella.

Helen had another proviso: Lawyer Seawell would become both president and chairman of the board of The Denver Post as well as chairman of the center's board of trustee.

Thus, it was that two weeks ago, while Seawell was east on business, a coup was attempted to oust him from the center he has all but completed. After 15 of the board's 17 members lined up solidly behind Seawell, President H. Lee Ambrose and Executive Director Richard D. Collins resigned.

Some think the rebels-fourth-generation Denverites-had been counting on support from a longtime Denver Post competition, The Rocky Mountain News, flagship of the Scripps-Howard Chain.

"That was stupid," remarks News Editor Michale Balfe Howard. "Don't they know that the center's the greatest thing that's happedened to Denver since the city bought its own water rights? They misjudged his steel."

Seawell's attitude toward the failed coup lay in his reaction to a cartoon the nest day in the Rocky Mountain News, which affirmed to some that Seawell was pushing the center too fast, but remained others of his long campaign. It was a drawing that could be viewed from two sides.Ever the optimit, Seawell took it as a compliment to the push he initiated nine years ago when he phoned Mayor Bill McNishols and said that if the city would provide the land, the Bonfils foundations would build the center. He plans to hand the original cartoon in the center's board room as a reminder that the going hasnht been all easy.

Besides its main stages, the theater complex has two other auditorims, a "black-box" studio, which can seat 200, and a cinema, which can seat 260, to be operated in conjunction with the American Film Institute. On its national advisory board, along with Roger L. Stevens and Arthur Goldberg, is Douglas Fairbanks Jr. Who has promised to open this with a series starring his Denver-born father.

Artistic director of the $12-million Theatre Comples will be Edward Payson Call, who has staged over a hundered productions in some 60 theaters from New York to Australia, including two for Arena Stage, "Catsplay" and "The Front Page." Raised in Georgetown and a University of Maryland alumnus, Call started as an actor and director at Laurel's Avondale Playhouse 25 years ago.

After four months in Denver, Call reports that hehs been studying it audiences and mulling his first production choices: "a spectacular for the first night and two smaller productions opening in the split theaters the next night."

When Call starts rehearsals in the fall for that New Year's Eve opening, the Helen Bonfils-Don Seawell dream will be up to him.

Whatever happens, the theater, of solid concrete and steel-supported glass, will survive. "It would cost more to tear it down," chuckles Seawell, "than it did to build." CAPTION: Picture 1, The Denver Center's 1,600-car garage, center roof and facade of the 1907 Auditorium Theater; Picture 2, its chairman, Donald Seawell plans to hang the cartoon in his board room as a reminder that the going hasn't been all easy.

by Michael Collins and the Denver Center of the Performing Arts