After the installation and activation of Thea for Rhino there are a couple of steps that need to be done to have a better workflow when working with the plugin.
From the main menu select Render > Current Renderer > Thea for Rhino. When the plugin is the current renderer, you will be able to see the new Thea Material in the Materials tab.
By default, when rendering interactively in the viewport, surface isocurves, mesh wires, etc will be displayed. To disable them, go to the main menu Tools > Options > View > Thea Render and from the Visibility rollout uncheck all the parameters.
To make the Thea Render settings tab visible, right click on any Rhino's tab and select 'Thea Render' from the list or use the main menu: Thea Render > Render Settings.