I had been customising Sharepoing theme lately in 2010 version, and i have to say it’s not very fun, mainly due to lack of documentation online and a big change in UI elements comparing to version 2007.
Sharepoint 2010 allows you to optionally upgrade your UI to version 4, which is the new 2010 version or stays as version 3, which uses the 2007 style packages. What was good with 2007’s was that the style packages actually exist in the 12 hives folder, meaning you could actually go and open up the stylesheets and work from there. However with version 4, things turn ugly. Microsoft used the Office theme package (.thmx) to do its styling in Sharepoint.