How do I add my css

This guide will help you to inject your CSS code: How-to-Inject-Custom-CSS-Codes

We have a Designer tool as well that will help you to add your custom CSS code, to get this till you only need to click on the designer icon, it is placed on the top toolbar while in the form builder: 

Here you will be able to paste your custom CSS code: 

This link will provide you with some videos about designer as well: -Form-Designer-Tutorial-Let-s-create-fantastic-forms-

Hope this helps.