Integrity, proportion, radiance: does Aquinas have a theory of beauty or just three words?

In a seminar on Aquinas we hit the passage everyone quotes on beauty: Summa theologiae I, question 39, article 8, where he says beauty requires three things: integrity or perfection (things that are lacking are ugly for that reason), due proportion or harmony, and clarity, which he links with bright colour.

Elsewhere, in question 5, he gives the even more famous line that beautiful things are those which please when seen.

Our lecturer pointed out two things that unsettled me. First, the three conditions appear in a discussion of the Trinity, where Aquinas is explaining why beauty is appropriated to the Son. They aren't a stand-alone aesthetics. Second, Aquinas never wrote a treatise on beauty, and a lot of what we call "Thomist aesthetics" was assembled later. Umberto Eco wrote a whole book reconstructing it. And most of my classmates first met the three words in Joyce's Portrait of the Artist, where Stephen Dedalus builds a theory of art out of them.

So my question: is there really a theory here? What does claritas mean: literal brightness, or intelligibility, the form shining through? And does "pleases when seen" mean beauty is in the seeing, or in the thing seen?