Here you can define any CSS classes required to style the appearance and design of the web part. The specified styles will be loaded by pages where the given web part is placed. Please note that this requires the Allow CSS from components setting to be enabled in Site Manager -> Settings -> System -> Performance.
Any changes must be confirmed by clicking the Save button in the tab's header. The Preview button allows you to edit the stylesheet side‑by‑side with a preview of how the web part appears on the live site. Please see the Design preview help topic for additional details about the preview mode.
Additional information can be found in: