How to set a theme for a report design
In Resource Explorer, expand the library that contains the theme. Expand the Themes node in the library.
Drag the theme that you want to use from Resource Explorer and drop it in the layout editor.
Any predefined styles in the theme take effect in the report design immediately. The appearance of any report items in the layout editor changes to match the styles in the theme. The library appears in the Libraries node of the Outline view.