In the past I covered 2 articles on the new SharePoint 2013 Composed Looks and Preview Files that mention theming and how it works:
In those articles I talked about the new theming engine and how to create your own new Composed Look which was pretty cool. I also mentioned a tool that would help us build our SPCOLOR file or basically our theming file.
Our goal is to provide new color possiblities or “palettes” for the power user to choose from. Don’t forget that you can place these anywhere you want when building your own custom master page. Themes are not just for the out of the box look and feel. If I create my own Master Page and decide to allow the Power User to change the color of the top bar only, I can.
The new SharePoint SPCOLOR file builder tool
I have been waiting for this for a while, it was announced during a session at SPC12 in Vegas but never saw the day of light. This should be in everyones SharePoint Install in my opinion.
You can download it here: http://www.microsoft.com/en-us/download/details.aspx?id=38182