The article covers the ins and outs of actually creating your own theme in SharePoint. To get access to the article you will need to create a member account, but the process is fairly painless. Here is the intro paragraph:
When it comes to branding in SharePoint, few things are as important as the effective use of Cascading Style Sheets (CSS). This is because CSS is the primary mechanism that Microsoft gives us to override their out of the box look and feel. One excellent way to use CSS to style SharePoint is to use a theme. SharePoint themes take advantage of the CSS concept of “cascading”. The basic idea is that styles that are loaded last on a page typically override styles of the same specificity that were loaded earlier on the page. SharePoint themes are mostly made up of CSS that is overriding already defined out-of-the-box SharePoint branding. That familiar light blue and white UI can be changed to look radically different just by creating a custom SharePoint theme and applying it to a site.