Represents a theme in the library
Theme is a container for a collection of Styles.A library can define a number of theme, themes in a library share "theme" namespace.
When a report use a libary with themes, report can specify one theme. In that, all the styles within the theme is availble to the report, and the selectors within the theme is put in the search path for a style property.
A collection of styles within the theme.
Each theme can contain zero, one or more styles.