While themes and thematic statements crop up throughout our stories, we can consciously weave them into our work to drive the desired effects in our readers. Once we make the choice of what themes and what thematic statement we want to write with, we can start layering them into our works, enhancing both our stories and our themes.
Like thematic statements, themes aren't necessary, though readers may find themes in your work you did not intend. To help steer your story in the right direction in the reader's mind, elect specific themes you want to explore in your story. Now, a theme can be any idea like love, dating, alcoholism, fighting, capitalism, or stoicism. The list goes on and on.
Multiple thematic statements will cause a lot of confusion both for you as a writer and for readers trying to understand the story they are experiencing. Stick to one powerful thematic statement, deepened and developed by connected themes that show your thematic statement at different levels and aspects.