A theme is a set of design elements and color schemes that define the visual layout of forms. Themes provide a consistent, professional look and feel to your forms.
Once a theme is associated with a site, it is automatically applied to all existing forms within the site and to any new forms added to the site.
When you select a predefined style for your theme, the new look is applied to all forms using that theme.
You can also configure themes for form controls. Any settings for the controls at the form level override the site-level settings in the following order: control-specific styles, font/background overrides, and style sheet used in a theme.