What are some ways to establish thematic threads throughout an application?

Hi, all! I'm technically not yet in high school, just very paranoid about my future—I don't know how that affects the urgency of this post—but I think this is something that'll ultimately be helpful to others in these forums. Through some perusing of advice blogs from the internet's college counselors, I found a general consensus to be that developing a sort of 'theme' through your apps (a general outline of your character or academic interests, I think) is generally helpful. I feel like that makes sense, especially considering admissions officers' hunt for 'character'. However, I'm confused as to how this is done. Is there a way to create a potent theme or is it mostly the part of the officers' intuition?