Themes I am talking about are like the theme of 'love', 'fate', 'duality' and 'conflict'
Tools I am talking about are 'foreshadowing, irony, conflict, colorful language, etc'
Love vs. Hate is probably the most common (and obvious) of the themes written about in reference to Romeo and Juliet. However, one way in which you could develop that theme is to connect that theme to the symbolism in the play, specifically how images of Light and Darkness are used. For example, all the fights occur during the day, showing the reality of the situation in which the "star-cross'd lovers" find themselves, specifically the "hatred" amongst them. However, their meetings mostly occur in the nighttime (darkness), symbolizing the secrecy of their relationship, connecting then to "love" in the theme. Also, both of the lovers refer to each other in terms of light: Juliet says that she wants to "take my Romeo, and cut him out in little stars, and he will make the face of heaven so fine that all the world will be in love with night and pay no worship to the garish sun" (Act 3), therefore describing him as "light" in her life; Romeo says that "Juliet is the sun" (Act 1) at the beginning of the balcony scene, again describing her as the "light" in his life. The difference here then is that, although they must meet in darkness and secrecy, they yearn to be each other's light, a.k.a., their reality.
Since themes are generally meant to be more complex than one word, I would say "The presence of love in Romeo and Juliet creates not only conflict, but creates unneceessary mortality." I don't know it's whatever you want to use.
And then for literary tools foreshadowing is a really good one, but I think if you went more in depth to your paper, such as the listing of rhetorical strategies (parrelism, juxtapostion...etc.) You probably would get a better grade if you have already learned more complex literary tools. And if you haven't learned rhetorical strategies I would use the tools you have already listed.