Here's a short message from Jason Schneekloth, Microsoft program manager for the Theme Builder:
The Open XML Theme Builder tool provides a simplified user interface for creating custom themes which can be used to define the look and feel of documents created within Microsoft® Office PowerPoint® 2007, Microsoft® Office Excel® 2007, and Microsoft® Office Word 2007. The tool gives users the ability to quickly and easily design the look of a document without having to understand the inner workings of XML.
This is the latest rendition of the program we used to create the great themes that shipped with Office 2007. The tool lets you open a theme, adjust it's elements, test it, and save it out.
For example, here's the Urban theme loaded in the tool. Notice how the color elements are arrayed so you can see how, independent of the presentation design, the colors work together or where there may be problems.