You would change it in the CSS Form Builder wizard. When you initiate Form Builder - either though insert a new CSS Form or you are editing an existing form, you will see the CSS Form Builder dialog box pop up above Dreamweaver.
In the dialog box, you have the option to edit the elements in a form or to edit the style (look and feel). You want to edit the elements so you can select which element to modify.
Check out the getting started guide here: css-form-builder/
You will see a PDF for CSS Form Builder under "Getting Started".
In that PDF, there is a page marked "Modify individual form elements."
That will tell you how to get to each element to modify.