Antonyms of theme :
aside, digression, excursion, interjection, parenthesis, tangent.
Examples of usage:
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.
The theme is unpleasant; the author chose it at her peril. "Views and Reviews", Henry James.
Roger led his companion to talk; and the theme Which he chose, was herself, her life story. "Three Women", Ella Wheeler Wilcox.