Creating a new SharePoint theme is a fairly straightforward process. You can entirely leverage the existing themes as a foundation on which to begin constructing your own.
To make a new theme, simply follow these steps:
Navigate to the themes folder: C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\THEMES. Make your own theme named “EXAMPLE” by copy/pasting any existing theme folder.
Open your new Example theme and rename the INF file to EXAMPLE.INF
Locate SPTHEMES.XML at C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\TEMPLATE\LAYOUTS\1033. Add a new Template entry for your EXAMPLE theme, following the existing format. Replace the image thumbnail and preview references with your own images if you have them available.
Perform an IIS reset or an Application Pool Recycle on your server for the new theme to appear on your SharePoint site.
Increase business process efficiency and collaboration with Microsoft SharePoint
Note: It is important to follow the case-structure of folder and file names for your new theme to be recognized. For example, if you name your theme folder “Example” instead of “EXAMPLE”, your new theme will not show up. Same goes for the INF file.
Tips: Because you must perform an IIS reset or AppPoolRecycle for any theme changes to appear, I generally find it more productive to build a new theme in SharePoint Designer, using the Alternate CSS option, and then packaging the custom theme once it is finished.