You can use your own CSS to customize the look of your form. Styling forms with CSS is a great opportunity to add your company’s brand on the form, not to mention it offers a lot of flexibility during creation.
All you have to do is to go to Design → Add custom CSS and paste the code or enter the URL to the CSS file.
The changes will automatically take effect on your form. Hit the View icon to see the results.
Learn about Full CSS Form Customization.