Themes are subjective in nature. If you asked a hundred people, even Caesar scholars, for the theme of The Tragedy of Julius Caesar, you might get a hundred possible answers.
A theme should explain the meaning behind a work in its totality. It can’t just explain Act I, or the climax, or any part of story while ignoring the other parts. It should also be a statement about human life that is universally true; that is, it should be true for people everywhere.
With that in mind, we need to look at the play as a whole. Since Caesar dies halfway through the story, the theme shouldn’t be solely about his actual attempt to grab power, because that’s over at that point. I think we need to look at Brutus; he was the character that really moved the action, for better or for worse.
Brutus’ goals and actions are based on his conviction that he should be “honorable.” We see this trait in Act III, Scene II, as Brutus speaks to the people following the assassination of Caesar. He appeals to his, and his audience's, sense of honor:
Believe me for mine
honor, and have respect to mine honor, that you may
However, his attempt to be honorable does not guarantee political or military success. In fact, it actually helps bring about his downfall, because “honorable” decisions like not killing Antony and allowing him to speak at Caesar’s funeral ultimately work against him.
With this in mind, I would say the theme to The Tragedy of Julius Caesar is: Being true to one’s moral convictions does not guarantee worldly success. We see this in the fact that Brutus is true to his moral convictions, but he fails to accomplish his goal.