Click on Add custom CSS and Use the Code tab to type the CSS code or the URL tab to provide the URL to the CSS file located on your server.
If you choose the URL tab, make sure that the URL leads to the content of your stylesheets. Here’s an example: https://www.mydomain.com/styles/stylesheets.css. If the URL does not contains the .css extension at the end, the CSS styles will not be applied to the form.
Here is a list of a few custom CSS samples which you can use to style your online forms.