The interface for the theme editor is not really good right now, even though it is fully possible to create themes using the editor.
When you select your theme in the inspector, you should see a preview window. At the top of that preview window, you can see a Theme button that, when clicked, makes a dropdown menu appear. In it, choose Add Class Items then choose a type (like Button). Then choose Add, and you will be able to edit button properties in the inspector.