The viewer does permit, however, the setting of various global Graphviz attributes. This is equivalent to adding a node or edge statement to the graph. Such changes will affect any node or edge that does not set the attribute explicitly.
To set attributes, click on the Attributes button at the top left of the graph window. This will open a new window, listing all of the known supported attributes and the current global value. Initially, the window displays graph attributes. By clicking on the circle or the slash at the top of the attribute window, you can get a list of the default node or edge attributes. Clicking on the leftmost icon with the three connected circles will take you back to the graph attributes.
If you click on an attribute, you will see a brief description of its role at the bottom. To alter an attribute value, simply double click on the value you wish to change. This will open a text edit window, in which you can enter the desired new value. Pressing the return or tab key will store the new value, and update the graph drawing to reflect the change. More information about Graphviz attributes is available here - Node, Edge and Graph Attributes