What style of writing has been used by Saki (H. H. Munro) is his story "Dusk"?What is the kind of language and literary techniques used, etc. ?

While Saki's techniques are conventional, his sympathies are not.  In his short story, "Dusk," as in many of his writings, he satirizes the same people that he has entertained with his writing.  Norman Gortsby is a gentleman of some wealth whose complacency in his judgments of people is satrized.  As Gortsby sits on the bench in the park, Saki describes the unsuspecting man with irony:

He was in the mood to count himself among the defeated.

The ironic twist at the end of the story in which he believes that he has been proven wrong about the young man, priding himself of his graciousness in returning the soap and making him the loan of a sovereign, clearly pokes fun of his attitudes as the old gentleman returns as the actual owner of the soap:

Poor boy, he as nearly as possible broke down," said Gortsby to himself.  "I don't wonder either; the relief from his quandary must have been acute.  It's a lesson to me not to be too clever in judging by circumstances."

Even when Gortsby humbles himself and displays compassion, Saki's satire is unrelenting.  For, he retraces his steps to find the elderly gentleman search for his lost soap, thus providing readers with the surprise ending that the young man is, indeed, a swindler.