The third tab in the configuration is about design settings.
There are multiple designs available for the buttons. By using the drop-down box you can choose a button template. The selected design will be shown below the drop-down box. Do not forget to save the changes in order for the new design to become effective.
You can either use your website's font as specified in your template's CSS or use a Google Font from a dropdown.
Next to the button template, also the size of the buttons can be selected. If appropriate, you can select rounded buttons in stead of the square ones.