New York ? Don’t remember much about high school biology or physics. Couldn’t tell ya how to compute a calculus problem. But, for the love of Will Smith, the theme song to “The Fresh Prince of Bel-Air” remains fresh in the mind.

Smith’s catchy rap opened each episode of his hit ’90s sitcom, in which he starred as a street-smart teen from Philly who moves in with wealthy relatives. A whole generation knows it by heart – that, and the “Saved by the Bell” song.

TV themes, from “The Beverly Hillbillies” to “The Brady Bunch” to “Cheers” to “Friends,” conjure up memories of cozy nights, childhood bliss and a universal nostalgia for bygone days. But, today, show themes are doing a fast fade as the networks crunch their programming budgets.

Are they about to join the variety hour in the TV graveyard?

“It’s a rarity today,” TV historian Tim Brooks said of the catchy, tuneful opening. “It’s kind of like the Broadway musical producing hit songs – it just doesn’t do that anymore.”

Back in the day, even into the ’90s, shows usually had a “main title,” a 40-to-60 second opening montage that introduced the cast and was often set to music written by a composer, said Jon Burlingame, author of “TV’s Biggest Hits,” a history of themes. Songs summed up what a show was all about, whether spinning the tale of how a group of wacky castaways ended up on “Gilligan’s Island,” telling how a spunky single career woman was “going to make it after all,” or describing why six touchy-feely Manhattan singles were there for each other.

But now many sitcoms and one-hour dramas are dropping that device. They dive straight into the action, sometimes flashing the show’s title or logo at various points throughout an episode.

ABC’s “Lost” does it. The twisty drama begins after a teaser, which touches on what happened in previous installments, and cuts to a black screen at a crucial plot point. A white “Lost” logo swirls into view. Eerie music plays. The whole thing lasts about five seconds.

“That’s not a theme” nor an artistic statement, lamented Burlingame, longing for the urgency of the “Mission: Impossible” score.

Other title-flashers include ABC’s “Grey’s Anatomy,” which threw out its 26-second theme last year, and “Desperate Housewives” and NBC’s “My Name Is Earl,” which both switch off between showing the full credits and the logo. New shows – ABC’s “Brothers & Sisters” and “Ugly Betty” and NBC’s “Heroes” and “Studio 60 on the Sunset Strip” – display only the logo.

“Almost all shows have music, but it’s generic, it’s scene-setting, it’s short,” said Brooks, who estimated that fewer than 10 percent have “traditional” themes that set up the show.

Clearly, brevity is key. No drawn-out intro or hokey theme. Networks don’t have time for that – and neither, prevailing TV thinking goes, do the country’s couch potatoes.

“Producers feel, rightly or wrongly, that that interruption, if you will, is going to lose viewers,” Brooks said.

“I think one of the things that has squeezed themes out is this relentless kind of move toward tightening everything, making it go right from joke to joke, from action to action, from shootout to shootout, so that you won’t press the dreaded remote control.”

Thanks to the elimination of commercials between the end of one show and the beginning of another, shows overlap before fickle viewers have a chance to channel-surf to Another Network. More commercials air within a show, making episodes shorter. Main titles and well-rounded theme songs and scores? Sorry, no time, no money.