noun

• (dated) An essay written for school.
  "Father Dolan came in today and pandied me because I was not writing my theme."
• (music) The main melody of a piece of music, especially one that is the source of variations.
• (film, television, video games) A song, or a snippet of a song, that identifies a film, a TV program, a character, etc. by playing at the appropriate time.
• (crosswording) An additional puzzle within the crossword, typically involving a set of non-standard clues or answers.

verb

• (transitive) To give a theme to.
  "We themed the birthday party around superheroes."
• (computing, transitive) To apply a theme to; to change the visual appearance and/or layout of (software).