One of the minor peculiarities of barbershop culture (as opposed to its various major peculiarities) is the way it uses the word ‘theme’. Generally, if you ask someone what a song’s theme is, they’ll either give a poetic or literary response – love, loss, nostalgia, that kind of thing – or point to the primary identifying melodic idea as you would to identify themes in a Beethoven symphony. But what barbershop judges (well, specifically Music and Presentation judges) mean when they say ‘theme’ is the primary musical element in that particular arrangement.
Now, this can be quite a useful question to ask. The term is odd, but the concept is serviceable. One of the main differences between a performance that sounds like people just obediently singing the notes and words and one that carries musical meaning is a clear sense of what the song’s main musical strength is. What a song is primarily ‘about’ does not always lie in the lyrics: if the thing that stays with you days later is the shape of the melody or the groove of the rhythm, then that should be your starting-point for interpretive decisions.
The thing about a song’s theme, then, is that it can stand alone. The main test for if a song has a lyric theme is if it makes sense to print the words on a tea-towel.