Atype of polyphonic composition based on one main theme is a

A polyphonic composition based on one main theme or subject is called a fugue.      

To add, in music, a fugue is a contrapuntal compositional technique in two or more voices, built on a subject that is introduced at the beginning in imitation and which recurs frequently in the course of the composition.

Normally it is at the top left, above where the music starts.