Using a single basic theme is one method you can employ to guarantee your essay's coherence and effectiveness. It forces you to organize your thoughts and ideas and helps keep the essay focused. Without a theme, your essay will simply look like a haphazard collection of facts thrown together to impress the reader. With a main theme, the essay will have a focal point upon which you will develop the rest of the paragraphs.
In conceptualizing a theme, you have to consider the main question, "What would you like to say with your essay?" For example, do you want the admissions committee to understand your choice of major and career in relation to your family background, or do you want to share your passion for your chosen field and your desire to achieve excellence in it? Whichever theme you come up with, this has to be unique to you as an applicant and should be related to your personal background, choice of major, and choice of career. The more unique a theme is, the more interesting the essay will be. In this sense, you can capitalize on personal facts you believe are noteworthy of exposition.