MOSS does not allow you to set a default custom theme to your sites upon creation. The most painful example is in mysites. There are articles that give you the full detail on how to get around this but here is the quick and easy way thanks to our good friend Scot Hillier and his SharePoint 2007 Features on Codeplex.
Download Themechanger from http://www.codeplex.com/features/Release/ProjectReleases.aspx?ReleaseId=2502
Install it on server
Find both WSP files and deploy.
Edit the Feature.xml file in 12\TEMPLATE\FEATURES\Themechanger
The last bit is to assign which site definitions should apply the theme.
You edit this in the Elements.xml file.
I added the line for mysites - SPSPERS#0
To see how to get a list of all site templates go here - http://stsadm.blogspot.com/2007/08/set-available-site-templates.html
Make sure you recycle your app pool for changes to take effect.
That’s it, should all work!!