You can build advanced custom themes that add or change CSS classes, map to specific parts of the UI, and change the structure of pages. The topics linked below describe how.
You'll need to be comfortable working with FreeMarker and Soy templates. Depending on what you do, you might also need to get a more thorough understanding on how the application is structured; editing one file sometimes requires knowing how a few others work, too.
The advanced topics describe how to use the admin console to create, map, and apply themes. (We strongly recommend using the out-of-the-box theming tool for making simple site-wide changes, which makes upgrades much easier.)
Note: Keep in mind that the advanced changes will require more work when it's time to upgrade your site. For example, some of the files you can customize depend on libraries or other theme files that might change with the next version. If you find that the changes you want to make require changes to the logic or code in the theme files, the Jive Software professional services team can help you.