It’s a Grinchified Christmas. The Grinch has been sighted in verdant buses coursing the streets of town, doing photo-ops, or Christmas tree lightings. A recent movie, Dr. Seuss’ The Grinch, is plastering its green face all over the place. And now we have Dr. Seuss’ How the Grinch Stole Christmas! The Musical, in for a nice long stay through Dec. 29 at the Merriam Theater.

It’s lots of loud, bright fun. The exiting crowd, kids especially, were all smiles. Wednesday at the Merriam, the stars of the night were Philip Huffman as the Grinch and Lily Tamburo as an adorable Cindy-Lou Who, who wins our hearts while she warms up the Grinch’s. But the real stars were the kids in the audience.

Equal parts Tim Curry in The Rocky Horror Picture Show and Gene Simmons (without the oleaginous nastiness) of KISS, with enough original Boris Karloff to steady the character, Huffman gave a sidesplitting master class in ad-libbing. He muttered, “I love it when kids cry!”, scowled at stray boos, stopped midline for pretend fights with the audience: “What? You, old lady? You want a piece of me?” He won everyone over to the bad guy’s side. One high point was the climax, when the Grinch tries to say, “Merry Christmas,” and just can’t: “Merry … Cracker? Merry … Cranapple?” The kids started shouting from their seats: “No! Christmas!” Get it right, will you please, Mr. Grinch? He looks out at us in exasperation, as in, Hey, look, I’m trying, gimme a second here.

Around since 1994, this musical has shown astonishingly hairy green legs everywhere it travels, a durable national/regional seasonal success. The Dr. Seuss tale aims squarely against commercialization and for sincerity (a central tune tells us “It’s the Thought that Counts”), and further against grumpy antisocial folks and for happy communities.

I must say, the Whos are, at least when we first see them, just as “Irritating!” as the Grinch says they are, “Loud!” in both attire and comportment. As the musical hurtles on, Eduardo Uribe as Papa Who, the wondrous Mimi Robinson as Mama Who, and audience favorite Michael L. Parks as Grandpa Who gave us reasons to like them, the main reason being Cindy-Lou. As Young Max, Jared Starkey is athletic and also an able spokesdog for the audience’s feelings, very sad about the Grinch’s Christmas hatred and much on the Whos’ side.

To be honest, the original tunes are not memorable. They set up the characters and themes, I guess. The two original 1966 tunes, “Welcome, Christmas” and the still-hilarious “You’re a Mean One, Mr. Grinch,” are what everybody came for. W. Scott Stewart, a kindly Old Max and a nice baritone, leads us in a crazy sing-along, and the kids belted out a joyous “Stink! Stank! Stunk!”

Nothing much mattered. At one point, a big Who larder stuffed with Christmas goodies collapsed with a frightening bang. Wow! went the crowd, giggling as stage workers cleaned it up, and the show went back to the silliness. Have yourself a green little Christmas.

Dr. Seuss' How the Grinch Stole Christmas! The Musical Through Dec. 29 at the Merriam Theater, 250 S. Broad Street. Tickets: $45-$115. Information: 215-893-1999, kimmelcenter.org.