Ellen Moody's calendar for NA(see the NA FAQs on the NA board) then we can work out exactly what Henry and Catherine saw at the theatre that evening.

Here is the relevant part of Ellen Moodys Calender:

Sat [Feb 17th]:
Catherine tries to visit Miss Tilney to explain; she is snubbed at door; in the evening she goes to the theatre, but sees no Tilney; he appears in a box at 5th act, she longs for forgiveness and he does come round to their box, so both are not too proud; while she talks with Tilney, he talks to General; it was then that General was led to believe Catherine was an heiress 12:83, 84; 30:212

In fact, they saw not one but two musicals.

Here is the listing for the evening of Saturday 17th Februrary 1798 at the Orchard Street Theatre:

Abroad and at Home
followed by
No Song, No Supper

See p172, The Orchard Street Calender 1750-1805 ,edited by Arnold Hare( 1977).

Abroad and At Home was a comic opera, in three acts by J. G. Holman who also an actor,with music by W Shield ,which was first performed on 19th Nobvember ,1796 at London's Covetn Garden Theatre.In fact, it played there 28 times that season.

No Song ,No Supper, was,again another comic opra , this time in two acts.The music was by the famous Englsih composer, Stephen Storace and lyrics by Prince Hoare.It was a slightly older opera than Abroad and at Home having had its first perfomance in London in 1790.

I have managed to find the outline of the story, which is as follows:

Crop the farmer has married a second wife called Dorothy, who has an amiable weakness for a rascally lawyer named Endless. During the absence of her husband, Dorothy provides a supper for Endless, consisting of roast lamb and a cake; but just as the lawyer sits down to it, Crop, with Margaretta, knocks at the door. Endless is concealed in a sack, and the supper is carried away. Presently, Robin the sweetheart of Margaretta arrives, and Crop regrets there is nothing but bread and cheese to offer him. Margaretta now volunteers a song, the first verse of which tells Crop there is roast lamb in the house, which is accordingly produced; the second verse tells him there is a cake, which is produced also; and the third verse tells him that Endless is concealed in a sack. Had there been no song there would have been no supper, but the song produced the roast lamb and new cake...

which all to me sounds just as implausible as most of the comic operas of this era!

You might be interested to know that Stephen Storace was an English composer, born in London in 1763. His father, Stefano Storace, an Italian contrabassist, taught him the violin so well that at ten years old he played successfully the most difficult music of the day. After completing his education at the Conservatorio di Sant' Onofrio, at Naples, he produced his first opera, Gl'i Sposi malcontenti, at Vienna, in 1785 where he became friends of Mozart( in whose opera Le Nozze di Figaro his sister, Anna Selina Storace, first sang the part of Susanna.)

He returend to England in 1787 .In fact, Storace attained his first great success in 1789, in yes, especailly for you,Jan, The Haunted Tower, an opera which ran for fifty nights in succession in London(a very unusual occurrance !).

No Song, No Supper was equally successful in 1790.The music to Colman's play, The Iron Chest, first performed on the 12th of March 1796, created even a greater sensation than The Haunted Tower.

Sadly, however,this was Storace's last work. He caught cold at the rehearsal, and died on the I9th of March 1796.