I think you actually meant to ask about a topic sentence rather than a theme sentence. A topic sentence is the very first sentence of a paragraph and introduces exactly what will be argued in the paragraph. It's sort of like a "mini thesis" in that every sentence you write in the paragraph is aimed at proving your topic sentence. A theme, on the other hand, is a central idea within a piece of literature, a main idea that underlies the whole work. For example, one theme in Romeo and Juliet is violent, passionate emotions vs. rational thought. But if what you need is to write a paragraph analyzing one of Romeo's characteristics then you are really analyzing characterization rather than theme. Characterization is how an author presents the character as a person, including the character's personality traits and characteristics. Therefore, it seems you need to construct a topic sentence for a paragraph analyzing one attribute of Romeo's characterization, rather than a sentence about a theme of the play.
In order to construct a topic sentence discussing one of Romeo's characteristics, you must first analyze enough of his characterization so that you can decide what point interests you most to talk about. There are lots of things to consider when analyzing characterization. Among those things are what does the character do; how does he think; what are his motives; does he make good decisions or poor decisions; what do other characters think of him? A good starting point can be the very first scene in which we see him introduced. What is Romeo like in this scene? What's he saying and doing? One thing we see is that he is moping about. He feels extremely blue because he is being rejected by Rosaline. Plus, the only thing he really says about Rosaline is that she's fair, meaning beautiful; therefore, it seems that he is mistaking real love, love that knows and values a person, for lust. So one thing we know about Romeo's characteristics is that he is very young and naive. In addition, he refuses to move past Rosaline's rejection, preferring to let it break his heart, despite the fact that his cousin Benvolio is counseling him to do otherwise, as we see when Benvolio says, "Be rul'd by me: forget to think of her" (I.i.227). Romeo's response is that he can't possibly forget how to think. Therefore, we know that another characteristic of Romeo's is that he places more value in his passionate emotions than he does in his abilities to be a thinking, rational human being. We could even say that he is a very irrational person, or even stubborn.
Once you have figured out some of Romeo's characteristics, you can then decide which one you want to talk about most and construct your topic sentence. For example, if we chose to argue that Romeo is an irrational person, we could say for our topic sentence, "One of Romeo's characteristics is irrationality." From there, you would write a paragraph proving his irrationality, such as the fact that he refuses to listen to counsel and allows himself to be governed by his emotions.