What is the collective noun for themes?

There isn't a widely recognized collective noun specifically for themes. However, you could use a more general collective noun depending on the context:

* A collection of themes

* A set of themes

* A suite of themes

* A tapestry of themes (if they are interwoven)

* A constellation of themes (if they are connected in some way)

You could also use a metaphorical noun:

* A symphony of themes

* A kaleidoscope of themes

Ultimately, the best choice will depend on the specific meaning you want to convey.