SketchUp allows the user to apply different materials to the Front and the Back of a surface.
Thea for SketchUp can use just a single material and reacts differently, based on how the materials have been applied to a surface in SketchUp.

In Image 1, we can see both faces of the model at the same time. White is being used to display the front face and green for the back.

Image 1

Material applied only to the back of a surface. (Model on the left)
In this situation, Thea will use the Back material for the front face too.

Material applied only to the front of a surface. (Model in the middle)
Thea will use the front material for both surfaces

Different material applied to front and back face.(Model to the right)
Thea will ignore the material that has been applied to the back surface and use the front material for both.

Image 2

The following image shows what will happen when the material has been applied to the back face, the front is absent and the ‘Use back face material when front is absent’ option has been disabled.
In this case, Thea will use the default white material for both faces.
You can find it in the Preferences window (Thea Tool > Preferences)

Image 3: Use back face material when front is absent (Disabled)