An article by Todd Bagsinki details the method of creating .thmx files and using them as SharePoint designer. Though the idea is simple but the output isn't always sweet. Mapping all the classes and images etc. isn't easy at all.
Raymund Macaalay visual guide on SharePoint 2010 has take a clever approach and here is what you have to do to get it done.
- Goto theme gallery <site>/_catalogs/theme/Forms/AllItems.aspx
- Download a pre-installed theme as sample.thmx file
- Get the theme builder free from Microsoft's connect portal and edit the theme so easily
- All done, simply save the .thmx file and upload back to site's theme gallery to use it.