However, you could use a more general collective noun depending on the context, such as:
* Set: A set of themes
* Group: A group of themes
* Collection: A collection of themes
* Series: A series of themes
* Spectrum: A spectrum of themes
* Constellation: A constellation of themes
You could also use a more descriptive phrase depending on the specific themes, such as:
* A tapestry of ideas
* A kaleidoscope of perspectives
* A network of concepts
Ultimately, the best way to describe a group of themes depends on the specific situation and the desired effect.