One theme in Act 5 addresses man's ability to change or control his own fate. We see this demonstrated immediately after Balthasar tells Romeo that Juliet has died. We especially see it in Romeo's line, "Is it e'en so? Then I defy you, stars!" (V.i.24). The irony in this moment is that Romeo has just had a dream about Juliet waking him from the dead that he has interpreted as a prophecy of positive things to come in the future. Hence, when he learns that fate has taken Juliet's life but not his own he feels that he wants to challenge fate; he wants to challenge fate's decision not to take his life by taking his own life instead. Thus, the word "defy," which can mean "challenge," or "resist" helps portray the theme of man's ability to change fate (Random House Dictionary).
A second theme found in Act 5 portrays the exact opposite--that fate is in complete control of all men. We see this theme portrayed in the second scene of Act 5 when we learn that Friar Laurence's letter to Romeo detailing the plan of Juliet's faked death actually never made it to Mantua. Instead, Friar John, who was asked to deliver the letter to Romeo on foot, was detained in a sick house that was quarantined. We further see this theme portrayed in Friar Laurence's lines to Juliet in the final scene, "A greater power than we can contradict / Hath thwarted our intents" (V.iii.158-159). In other words, Friar Laurence is saying that either God or fate has taken control of their plans and sabotaged them, leaving Romeo dead, which perfectly portrays the theme of fate having complete control over all men.
A third theme that we see portrayed in the final scene of Act 5 relays the total destruction that uncontrolled, violent, passionate emotions can cause, such as hatred. The feud between the Capulets and the Montagues is the ultimate cause of so many deaths, especially Romeo's and Juliet's. Prince Escalus especially blames Lords Capulet and Montague for so many deaths in his lines, "See what a scourge is laid upon your hate, / That heaven finds means to kill your joys with love!" (V.iii.303-304). Since Prince Escalus clearly blames Lords Capulet and Montague for the deaths due to their hatred and their feud, we see that another theme portrayed relays the dire consequences that spring from uncontrolled, violent, passionate emotions, such as hatred.