This theme uses the Simple Social Icons Plugin, which can be obtained here or by searching in Dashboard>Plugins>Add New.
In your Widgets menu, drag the widget labeled “Simple Social Icons” into a widgetized Area. – The demo displays them in the Nav Social Menu area. Configure the widget by choosing a title, icon size and color, and the URLs to your various social profiles.
Here is a screenshot of the settings used in the Gastronomy demo: