Gatsby themes provide a new abstraction layer for building Gatsby sites. Themes can come prepackaged with visual styles and functionality, making common use cases like blogs or documentation sites easier to build. Themes are configurable and can be overridden or extended through shadowing. This allows for greater flexibility and customization compared to plugins. Themes are intended to make Gatsby more approachable for newcomers and those with common use cases through higher level abstractions and prebuilt functionality.