Question: Is there a good source to download free SharePoint Themes (.thmx) ?

Answer: 
Couldn't find a site which hosts only SharePoint 2010 themes. Two good themese are here <http://www.topsharepoint.com/themes>
Themes can be created in PowerPoint, exported, and imported into Sharepoint. This will allow you to create styles, color palletes and fonts appropriate to your organization's style guides.