If you want to custom tune your forms then click and insert your own CSS using HTML classes you can check using browser developer tools.
Using "CSS" Field
Use the CSS field to enter your CSS to adjust the look of the whole form and its elements.
For example, if you have created a simple registration form:
You might want to add the following CSS :
The CSS above will convert the form into the following more colorful version:
Using "CSS Class" field
Sometimes you will need to use the CSS Class field to add your own class name to the form. This can help, for example, to reuse the same CSS for different forms of similar type.
For example, for a popup form:
Or you can only give the class name to the form, but your CSS using this class can be directly embedded to the page where the form is used.