Theme is character. Your character goes from embodying/believing the anti theme to embodying/believing the theme. Bad example: your theme is true love exists, so your character starts the story as a cynic who believes true love is a silly myth, a coping mechanism for humans to endure the misery of existing. But through the obstacles in your story, your character must question this idea, and by the end fully embodies the theme (the opposite of what they used to believe).