Not every anthology is themed, but many are. Most will also be restricted to one genre (romance, sci-fi, fantasy, crime) or a particular location, like a city or a place such as a coffee shop, etc. This is because theme alone isn’t enough to unify a book encompassing ten or more stories that are widely divergent in genre.

The concept of how to create a cohesive anthology was explained to me this way:

Consider a community art project where you ask five local artists to paint murals of cats to be displayed in the local community center. You will get five wildly divergent styles, and clashing colors, and cats that don’t go together well. But, if you ask them to paint black cats (or orange or red), your community wall art will have a consistency that celebrates the variety of the artists’ different styles rather than an eye-bleeding jumble of…