SharePoint 2010 offers themes like SharePoint 2007 but this time the way you create and deploy a theme has changed. In SharePoint 2010 you don’t need to know CSS in order to create a theme and you don’t need to edit .xml files to deploy a theme.
Creating a theme
SharePoint 2010
If you want to create a theme for SharePoint 2010 you need to use PowerPoint. You can reuse a theme in your corporate colors and save it as a .thmx file.
SharePoint 2007
In SharePoint 2007 you need access to the server itself where you need to do the following steps:
Go to “C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\Themes” and create a new theme by making a copy from one of the others. Rename the .INF file in your new theme directory. Modify the CSS of your new theme. 
Deploying a theme
SharePoint 2010
Go to site settings and select the theme gallery to upload a new theme.
SharePoint 2007
Go to “C:\Program Files\Common Files\Microsoft Shared\web server extensions\12\TEMPLATE\Layouts\1033\SPTHEMES.xml” (or a different language) and include a reference to the new theme.
Using a theme
SharePoint 2010
You can select the uploaded theme if you go to Site Settings –> Look and Feel –> Site theme where you can select it. SharePoint 2010 offers you to update only the theme of the current site or the current site and all sub sites.
SharePoint 2007
You can go to Site Settings and select a theme which is almost the same procedure as in SharePoint 2010. The big difference is that themes in SharePoint 2007 only apply to the current site and can’t be inherited to sub sites.
Summary
If a site owner wants to change the color and font of a site then using themes is the way to go. If you want to change font-size or spacing you should use CSS or create a new Master Page. But be careful, the result may be ugly ;)
