There is no definitive list in the Machform documentation; the only way to see what CSS classes are available for customizing form fields is to look at the code. In the file view.css, search for /** Built-in Class **/. The only ones that you need to type into the Custom CSS Class field are:
- column_2
- column_3
- column_4
- column_5
- column_6
- new_row
There are others defined in this section, but those can be applied with options in the form editor GUI (for example, hidden).
You can also add your own custom CSS classes to a theme, and then apply your custom classes to individual fields by using Custom CSS Class field.