The Open XML Theme Builder is a tool that helps with the creation of theme files (*.thmx) that encapsulate the visual appearance and formatting of a document in order to help create a better looking document. So it helps in creation of custom themes or modify the existing ones. Theme files can be used by Microsoft Office PowerPoint 2007, Excel 2007, and Word 2007.
Source : meraTechPort
Download : www.codeplex.com/openxmlthemebuilder