Shared style is an equivalent of the CSS class that can be defined once and utilized across different web page views. Shared styles do not replace or restrict the usage of standard CSS stylesheets but provide a convenient shortcut to define and use them in a "quick-and-dirty" way.
Create shared styles
From the Shared styles page, click on the plus button to add a new style. Specify the name and its CSS rules.
You will see the list of available styles in the table view:
Use shared styles
Go to the Web pages editor. In an element Properties ⇒ Appearance, select the Shared styles from the dropdown list.