It’s more of a historical art movement than a theory. It could fall under the formalist theory or possibly even the linguistic one, depending on the artists’ intent.
In western fine art, there wasn’t really art for art’s sake. Most art was religious because the church could afford to commission artwork. Most of the congregation was illiterate, so the church wanted art to tell stories from the Bible and bring people closer to God. Architecture was very important during this art period for that reason. In China and Japan, landscape paintings were painted on silk and focused on longing and nature which reflected ideals from Buddhism and Confucianism. I can’t speak for commercial art since that’s not my background. Gutenburg’s printing press was invented in the 1400s, and that would eventually effect illustration…but it might not have been until after the Middle Ages.
