Themes are a type of GeneXus object that allows separating the controls design details from the development.
The task of defining how the controls are designed (what they look like, their font, etc.) is usually delegated to designers. So, when they provide that information you have to specify it inside a Theme. Each Theme is composed of a group of classes corresponding to GeneXus controls.
By default, every Knowledge Base offers some Themes automatically created.
The Themes are offered under the 'Themes' node present in the KB Explorer.
You can create a new Theme by right-clicking on the 'Themes' node or by selecting in the main GeneXus menu File > New Object. Through those two ways, you can create both Web Theme objects and Theme objects.