Are themes good or bad?
That depends on whether you are talking about a theme for your application or for your essay. For the former, bad. For the latter, good.
If you are applying to a top program, you don’t want your application to have a nice, neat theme that simply labels or tags you: The Geek, The Actor, The Compassionate Soul. The Soldier Hero. No matter how nice the theme, if your application presents you as a uni-dimensional human being, you are making it easier for a school to reject you and harder for them to recognize your uniqueness and sheer humanity. Human beings aren’t neatly tagged or easily labeled. They are complex and multi-dimensional. If your application presents you well, it too will be varied and textured.
Themes in essays, however, are not only good; they are necessary. Your application essays and personal statements, like all other pieces of communication, must have a point: the message that you want to convey. Your core message — that central kernel — is your theme. An essay without a theme may say nothing or be hard to follow or try to cover too much ground or all the above. A good theme can structure you in writing your essay and guide your reader when reading your essay.
If your application has more than one essay, each essay should have a theme that complements — not duplicates — the themes of the other essays. Together those essays work to present you as an individual with diverse interests and multiple talents.