If you view a tersus project in the Navigator view (select Window>Show View>Navigator), you will find the CSS files under the web/ folder. Look for files with suffix .css or .less.
You can modify the content of these files, add content to any of them, or add new f.css and .less files.
Examples of using CSS style classes to control the UI can be found in several forum posts (e.g. http://www.tersus.com/#Id=6277 and http://www.tersus.com/#Id=8383).
For best results, use the Firefox browser..