You can use your own CSS for styling forms. To start, go to the Design section of your web form and scroll to the bottom, where Add custom CSS button is available.
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 to Load CSS from URL, 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 contain the .css extension at the end, your CSS styles will not apply to the form.