Themes in SharePoint are tough. I think this quote describes it nicely:
If you have attempted to override the styles on an out of the box SharePoint site, you know that it isn’t a very easy thing to do. The core.css file that contains the basic rules has 979 different style rules.
The core.css file uses a palette of 132 colors and 143 images.
The default page of a newly provisioned team site uses only 61 of those rules.
So, unless you have branded a lot of SharePoint sites and are intimately familiar with core.css and the default master pages and page layouts, just figuring out where to start modifying can be a daunting task.
So Doug has built SharePoint Skinner to make that easier. I’ve not tried it (yet) but I like his thinking…