.thmx

Theme that can be applied to a document or presentation created with Microsoft Office 2007 or later; contains pre-defined colors, fonts, and formatting; typically used as a template for creating multiple files with a similar look and feel.

Unlike previous versions of MS Office, a single Office 2007 Theme can be applied to Microsoft PowerPoint, Word, and Excel documents. This makes it possible to create a uniform look across multiple types of documents.

Custom themes can be saved by selecting "More..." in the Themes Group panel, then choosing "Save Current Theme." Saved themes can be loaded from within the Themes Group inside the Page Layout tab of Word, Excel, and PowerPoint.

Important: THMX files are also used by Microsoft SharePoint for setting themes.