The PS should always end on a positive note, and the conclusion should summarize the body of the statement. It is important to reiterate the primary message of the personal statement and thank the audience for their consideration. This will leave the reader with positive images of the applicant as they put the personal statement down.
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.