Definitions of theme:

• noun:   a unifying idea that is a recurrent element in a literary or artistic work

  Example: "It was the usual `boy gets girl' theme"

• noun:   (music) melodic subject of a musical composition

  Example: "The theme is announced in the first measures"

• noun:   the subject matter of a conversation or discussion

  Example: "His letters were always on the theme of love"

• noun:   an essay (especially one written as an assignment)
• noun:   (linguistics) the form of a word after all affixes are removed
• verb:   provide with a particular theme or motive

  Example: "The restaurant often themes its menus"