There are two ways to set a theme for an issue:
Create new issue
When you create a new issue From Design you can choose from a selection of themes. Selecting an option from the Theme drop-down list will display a preview of how an issue will look with the theme applied.
When you are creating your issue in TigerDesigner the theme can be updated by selecting Document Tools followed by the Select Template Theme option.
Pre-designed publication styles are set up within the system which complements each theme.