In SharePoint 2010, we are able to use any of the pre-built Themes or customize any of them using just the browser. If that was not enough , we are able to use either Microsoft Word, PowerPoint or Theme Builder to create new .thmx files that could then be uploaded to the Theme gallery by the site collection administrator and made available for site owners and designers to use on their sites.
The whole theming engine has changed and been reworked in SharePoint 2013. Everything is based on HTML instead of any proprietary format. The image below shows the actual Theme Gallery which is filled with font and color palette files.
As a result of this change in direction for theme building, you are no longer able to use Word, PowerPoint or Theme Builder to create new Themes.
Themes are only modifiable using the internet browser (aside from programmatic methods of course, which we will not be discussing here). Fourteen HTML 5 based Themes are available out-of-the-box to be used as needed. When designing a theme in the browser, you can pick any of the provided Themes as a starter template then design a much richer customized theme by choosing the fonts, color palette and your own background image.
The video tutorial below shows the whole process at work:
* More SharePoint 2013 videos available here.