Materials in Thea for Rhino can be created in the same way you create Rhino materials.
To be able to see the new Thea Material option, Thea Render needs to be the active render engine.
From the top menu select: Render > Current Renderer > Thea Render
There are two ways to add a new Thea Material in the material list:
More details on content browser, here.
Thea for Rhino offers two ways to edit Thea Materials. You can either use Material Presets as a starting point or use the Advanced Material editor to have complete control over the material parameters and make use of the advanced layering system that Thea makes use of.
To start editing a material, select it from Rhino's material panel and click on the "Edit" button. This will bring up the Thea Material Editor window with the option to use it either in Preset or Editor mode.