Suppose, with Aristotelians, that the good is perfection. Thus, the good is always the good of some entity, and it consists in that entity's being in some way perfected—its nature being in some way realized. Now consider this problem: many cases of beauty are not cases of an entity being beautiful. A hole could be beautiful in shape, and the wave-like arrangement of air molecules could be beautiful in sound, yet it does not appear that the right ontology should include holes and wave-like arrangements of air molecules.
Now, it is always good that there be a case of beauty. We've seen that a case of beauty is not always a case of a beautiful entity, so we cannot simply say that this good is a good of that entity which has the beauty. What is it a good of? Not the perceiver—for an unperceived beauty is still good (if this is denied, the following either falls apart or needs to be redone almost completely).
Consider, for instance, a sand mandala. Suppose for simplicity (and contrary to fact) that each grain of sand is an entity, but we do not burden our ontology with bunches of grains. What entity's nature is in some way being realized in the beauty of the mandala? Is it that grains of sand, in addition to their more ordinary ways of being perfected, have in their nature a special way of being perfected, by being beautifully united with other grains? That is not a completely crazy idea, but I think only a theist is likely to think it. It requires a certain pre-established harmony. For just as the grain of sand has a nature that specifies the possibility of its being perfected by being put in a beautiful arrangement, so does a tiny colorful snail—and exactly the same arrangements can be made from tiny snails as from grains of sand, and they will still be beautiful and valuable. Thus, all physical entities have in common natures that are perfected by being put in certain beautiful arrangements. This is only plausible if the natures of things have a common origin, and a common origin to which value matters.
Alternately, we might suppose that the value of the beauty of the mandala is a perfection not of the grains of sand, but of the people who made the mandala. On this view, which is also plausible, the value of beauty is grounded in the nature of the artist. But if this is right, then unless there is a creator, natural beauty—the beauty of sunsets and nebulae—will be in trouble.
So it may well be that the idea that beauty has a value independent of observation requires theism.