Antonyms of theme:
aside, digression, excursion, interjection, parenthesis, tangent.
Examples of usage:
  1. But in the English novel of society, although the theme be marriage, it is not necessarily love.
    "From the Easy Chair, series 2", George William Curtis.
  2. The theme is unpleasant; the author chose it at her peril.
    "Views and Reviews", Henry James.
  3. Roger led his companion to talk; and the theme Which he chose, was herself, her life story.
    "Three Women", Ella Wheeler Wilcox.