Sharepoint themes can be used on free product such as WSS (Windows SharePoint Services) and on MOSS (Microsoft Office SharePoint Server) which is a commercial and expensive software technology from Microsoft windows.
There are tons of practical uses of using Sharepoint technologies as it includes instant collaboration, management components, sharing workspace so as to host websites.. Under the Sharepoint products there are different technology platform such as Office sharepoint server 2007, Windows Sharepoint service, Groove, Project server and so on..
The default Sharepoint theme can evoke boredom sometimes, so we can change it to some colorful and free-to-download Sharepoint themes. To install such themes, just put the files into the template/layout folder in the installation directory and browse the theme from Microsoft Sharepoint to activate it.
After browsing through the web for some excellent themes, I found Daniel providing free sharepoint themes on skydrive (download link) but make sure you test these templates on sharepoint testing environment.
Another Sharepoint theme collection comes from Microsoft who have recently provided 10 themes to download that can be found here. The size is around 7mb and contains 10 sharepoint themes which when unzipped can be implemented on your Sharepoint 2007.