All 3 Uses

theme
in
In the Time of the Butterflies
(Edited)

The theme was to be our centennial and the generosity of our gracious Benefactor.
Chpt 1.2 *

theme = unifying idea
So here's my final menu: (Bear in mind today is the Day of Lovers and so red is my theme.)
Chpt 2.7
The theme was the exploration of the meaning of Mary in our lives.
Chpt 2.8

theme = topic (of conversation)

Definitions:

(1) (theme as in: theme of the novel)
a basic idea that underlies what is being said or done -- especially in a literary or artistic work
(2) (theme as in: musical theme)
the initial or primary melody of a musical composition

or

(as in "theme song") music that is most repeated in a film, television series, or other theatrical production
(3) (theme as in: wrote a 2-page theme on my summer)
an essay written by a student

(This sense of theme is much rarer in recent decades than in the past.)
(4) (meaning too rare to warrant focus)