SharePoint 2010: Create a Theme using Microsoft PowerPoint - TechNet Articles - United States (English) - TechNet Wiki

1. Open Microsoft PowerPoint and choose (or create) a theme you like.
2. Click the “Design” tab in the ribbon
3. Then click the little triangle in the corner of the themes to display more
4. Click “Save Current Theme”
5. Choose a location and click save. Now you have got a .THMX file!

Great, now you’re thinking… this theme file has got to be either a CSS or XML file that I can easily edit. Sorry I tried to open it in Notepad and got gibberish with a few file references at the bottom.

1. Just go into Site Actions, click Site Settings
2. In the Look and Feel section, click Site theme
3. In the Select a Theme section select the theme you uploaded. 

1. The ability to easily create them in PowerPoint (isn’t that cool enough?)
2. Extensible (export and import)
3. Easy to modify existing 2010 themes
4. Applies to both System (Layouts) and Application pages
5. Designed for end users to be able to easily create and modify SharePoint styles using the browser