There are various theme options. Basically, you can change just about anything on your web form, from text font and color to the style of the submit button. The settings categories that you can tweak in the theme customization panel are: Logo, Background & Form Frame, Text, Highlight, Buttons, Errors, Others. In order to tweak these settings, you need to work on a copy of a predefined theme or on a blank theme.
Background & Form Frame
Use custom CSS