Refers to the main musical piece associated with a film, TV show, game, event, etc. Similar to 主題歌, but テーマ曲 can be instrumental or vocal.
See also: 主題歌
The theme music for this drama is very memorable.
The orchestra performed the movie's theme song.
Compound of テーマ (theme, from German 'Thema') and 曲 (musical piece).