Each Cargo site contains a CSS Editor where you can access the entire stylesheet of your site. You can modify existing styles as well as add your own custom CSS rules. Access it via the Design tab.
To change the style of an existing element — search for its selector in the CSS Editor (Cmd-F). To change Thumbnail Titles for example, look for
. You can also identify a selector by hovering over an element in your site while the CSS Editor is open..thumbnails .title
(Please note that Cargo offers limited support for customizations.)