See http://www.tersus.com/#Tag=CSS & http://www.tersus.com/#Tag=Client Design for details on CSS usage in Tersus. I recommend you start with http://www.tersus.com/#Id=1349.
With regards to the Theme Editor, the truth is we have we have virtually stopped maintaining it, as we are working on a new, cross-device and form-factor, UI framework, which the current editor architecture is ill equipped to support. I suggest you dedicate your efforts on the above instead, as CSS proficiency will be crucial in developing appealing applications.
For best results, use the Firefox browser..