The body of your personal statement is going to be hard work _ you need to use around 50% of your word limit to give the admission representatives concrete evidence of why you stand out from the crowd in this application process. The body is also where you will be answering questions and giving examples of experience and affects of experience. Answers and evidence need to keep reflecting who YOU really are. If you're struggling to keep under the word limit, allocate each question a word limit as well.
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.