By targeting the Gravity Forms Collapsible Sections CSS classes, you can easily customise the look of the section header and body. This demo customises Gravity FormsCollapsible Sections using the CSS shown below.
The CSS below was used achieve this style (the base font and text color are not defined here and are inherited from the theme).
Please note that this is an example of what you can do with custom CSS, more than a copy & paste solution. Feel free to use the styles but they will most likely require further customisation to suit your website and needs. Read our support article for more info