It's probably related to the ancient worldview where the solar system consists of seven spheres that loosely correspond to planets. Romans weren't cristians until the very late: they removed the statue of Jupiter from their capitol only because the Church wanted to erase all traces of ancient religions. Dodecahedron probably had occult meaning: "the entire skies are arranged according to this shape" and the circular holes must be those planets or spheres, so if one puts a candle in the middle, it would cast "planetary" images on the walls. This would have the symbolic meaning of arranging skies with dodecahedron, and the candle would be the sun.