In An Inspector Calls, what tone is set in the opening exchanges between the characters?

The opening scene of An Inspector Calls creates the sense for the audience that they will be seeing a traditional British living room comedy. Sheila is described as speaking "gaily," or happily; later she remarks that "this is the happiest day of my life." The atmosphere is playful, celebratory, and warm. Priestley embeds a hint of darkness through the use of dramatic irony in Arthur Birling's predictions for the future. A successful businessman with a strong capitalist sensibility, Birling claims that technological advances will surely erase the need for war. Priestley's audiences would have understood that technological advances did, in fact, play a major role in creating the horrors of World War I.
In another example of irony, Birling describes the ill-fated Titanic: "Why, a friend of mine went over this new liner last week—the Titanic—she sails next week—forty-six thousand eight hundred tons—New York in five days—and every luxury—and unsinkable, absolutely unsinkable." Of course, we know that the Titanic did, indeed, sink.
When the Inspector arrives, Priestly describes a shift in the lighting, with a "harsh bright light" replacing the traditional soft theatre lights. This indicates to audiences that the warm, convivial, and traditionally Victorian atmosphere was, in a way, artificial and is about to be exposed for its dark truth.