If you are running Windows XP SP2, please ensure that you have the .Net Framework 3.0 and the Office 2007 Primary Interop Assemblies installed prior to using this tool.
The tool lets you open a theme, adjust any of its elements, test it, and save it. While modifying the existing themes (* .thmx files) , found in the 'Document Themes 12' folder of the Microsoft Office installation directory, be sure to first make a copy of the existing themes before tweaking them so that just in case, you can have a backup.
You can download this tool from : http://www.codeplex.com/openxmlthemebuilder
From the same link you can also get the .Net Framework 3.0 and the Office 2007 Primary Interop Assemblies required if you are installing it on Windows XP SP2.