how we can express the guilt as a theme in the birthday party ?

Atonement

"One of the great ironies in this play is that it uses what appears to be a fairly undramatic, realistic setting which nevertheless hides a surplus of guilt. The theme of atonement runs throughout the play. Stanley's past is never detailed, but he is clearly a guilty man. He is vague about his past, and does anything to distract Goldberg and McCann. He does not wish to atone for whatever he did, but is forced to do so through torture. Goldberg, too, wishes to avoid whatever sins torture him but cannot fully escape them; his mood in Act III shows that he is plagued by feelings he does not wish to have. In the end, all of the characters are like Lulu, who flees when McCann offers her a chance to confess - everyone has sins to atone for, but nobody wants to face them."

thank you

You are very welcome!