What theme(s) is reflected in Act 3, Scene 1, when Mercutio is stabbed, and he says "A plague o' both your houses!" And he goes onto say, "Ask for me tomorrow, and you shall find me a grave man."

Mercutio's final lines in Romeo and Juliet represent several themes. On the surface, he is angry with the Montagues and Capulets because their feud has led to his death. He tries to hide the severity of his injury at first, but this turns when he says, "Ask for me tomorrow, and you shall find me a grave man." He is mortally wounded, and he knows he will be dead by the next day. The Black Plague, which struck at its worst not long before Shakespeare lived, could lead to a fairly quick death--though not as quick as being stabbed. The feud is the plague, in this case. It devours everything in its path, good, bad, or neutral.
The other piece requires an understanding of what the bubonic plague outbreak of the 1300s led to in England and elsewhere in Europe. The Black Death, as it was also known, killed almost two thirds of Europe's population (History Today). This left a vacuum which was ultimately filled by the rising middle class, ultimately leading to the prosperity, invention, and artistic/architectural boom known as the Renaissance, which was Shakespeare's era. The Renaissance was a rebirth; that's what the word actually means in English. So, in a way, Mercutio is not only foreshadowing and cursing the devastation of this plague, this feud, but also the rebirth that stems from it. At the end of the play, after Romeo and Juliet have claimed their own lives, Montague and Capulet end their feud and fair Verona grieves, but peace and hope are reborn. This is what Shakespeare promises with his Chorus, and Mercutio reminds the audience/reader of that promise when he dies in the middle of the play. It's as though he's saying, "I'm really angry that your ridiculous fight cost me my life, but I won't be the only one. At least though, after all this death and destruction, this world will get a little brighter."

Mortality is a strong theme in Shakespeare's Romeo & Juliet. Mercutio's famous words "A plague o' both your houses!" foreshadows the outcome. The two houses he references are the Capulets and the Montagues who have been locked in an endless feud. And where go the Capulets and Montagues, death follows. Mercutio blames both families for his death. Because he was Romeo's friend, he was Tybalt's enemy by default. His dying wish is for something terrible to happen to both families. He even blames Romeo for his death because Romeo distracted him by trying to quell the argument. He says, "you shall find me a grave man" because he knows that he is dying--one more victim of the long feud. The plague that soon after falls on the houses of Capulet and Montague is the deaths of Romeo and Juliet. Though they both committed suicide, their deaths are still a result of the feud between their families, and the grief that follows is no doubt the plague that Mercutio wished on them.