The Collaborative International Dictionary of English v.0.48:

Theme \Theme\, n. [OE. teme, OF. teme, F. th[`e]me, L. thema,
   Gr. ?, fr. ? to set, place. See Do, and cf. Thesis.]
   1. A subject or topic on which a person writes or speaks; a
      proposition for discussion or argument; a text.
      [1913 Webster]

            My theme is alway one and ever was.   --Chaucer.
      [1913 Webster]

            And when a soldier was the theme, my name
            Was not far off.                      --Shak.
      [1913 Webster]

   2. Discourse on a certain subject.
      [1913 Webster]

            Then ran repentance and rehearsed his theme. --Piers
                                                  Plowman.
      [1913 Webster]

            It was the subject of my theme.       --Shak.
      [1913 Webster]

   3. A composition or essay required of a pupil. --Locke.
      [1913 Webster]

   4. (Gram.) A noun or verb, not modified by inflections; also,
      that part of a noun or verb which remains unchanged
      (except by euphonic variations) in declension or
      conjugation; stem.
      [1913 Webster]

   5. That by means of which a thing is done; means; instrument.
      [Obs.] --Swift.
      [1913 Webster]

   6. (Mus.) The leading subject of a composition or a movement.
      [1913 Webster]

WordNet (r) 3.0 (2006):

theme
    n 1: the subject matter of a conversation or discussion; "he
         didn't want to discuss that subject"; "it was a very
         sensitive topic"; "his letters were always on the theme of
         love" [syn: subject, topic, theme]
    2: a unifying idea that is a recurrent element in literary or
       artistic work; "it was the usual `boy gets girl' theme" [syn:
       theme, motif]
    3: (music) melodic subject of a musical composition; "the theme
       is announced in the first measures"; "the accompanist picked
       up the idea and elaborated it" [syn: theme, melodic
       theme, musical theme, idea]
    4: an essay (especially one written as an assignment); "he got
       an A on his composition" [syn: composition, paper,
       report, theme]
    5: (linguistics) the form of a word after all affixes are
       removed; "thematic vowels are part of the stem" [syn: root,
       root word, base, stem, theme, radical]
    v 1: provide with a particular theme or motive; "the restaurant
         often themes its menus"

Moby Thesaurus II by Grady Ward, 1.0:

175 Moby Thesaurus words for "theme":
   IC analysis, accidence, action, affix, affixation, allomorph,
   anagnorisis, angle, architectonics, architecture, argument,
   article, atmosphere, background, background detail, basis,
   bound morpheme, burden, case, catastrophe, causerie, chapter,
   characterization, color, complication, composition, concept,
   concern, conjugation, continuity, contrivance, core, cutting,
   declension, decorative composition, decorative style, denouement,
   derivation, descant, design, detail, development, device,
   difference of form, discourse, discussion, disquisition,
   dissertation, enclitic, episode, essay, essence, etude,
   examination, excursus, exercise, exposition, fable, falling action,
   feature, figure, first approach, focus of attention,
   focus of interest, foil, foreground detail, form, formative,
   free form, gimmick, gist, head, heading, homily, idea,
   immediate constituent analysis, incident, infix, infixation,
   inflection, introductory study, issue, keynote, leitmotiv, line,
   living issue, local color, lucubration, main point, matter,
   matter in hand, meat, memoir, monograph, mood, morceau, morph,
   morpheme, morphemic analysis, morphemics, morphology,
   morphophonemics, motif, motive, movement, mythos, national style,
   note, notion, ornamental motif, outline, pandect, paper, paradigm,
   paragraph, pattern, period style, peripeteia, phrase, piece, plan,
   plot, point, point at issue, point in question, prefix,
   prefixation, preliminary study, problem, proclitic, prolegomenon,
   question, radical, recognition, repeated figure, research paper,
   review, rising action, root, rubric, scheme, screed,
   secondary plot, setting, sketch, slant, special article, stem,
   story, structure, study, style, subject, subject matter,
   subject of thought, subplot, substance, suffix, suffixation,
   survey, switch, term paper, text, thematic development, thesis,
   thread, tone, topic, touch, tract, tractate, treatise, treatment,
   twist, word-formation