I think beauty is definitely 'in the eyes of the beholder'.
Though culture plays part, I find that like minded people are inspired by similar things whether they live in China or Sweden.
In terms of historic eras - I believe the core values have been the same but the expression may have changed. Also technology, education and general awareness play a huge impact. E.g. it is no longer beautiful to wear a coat made from the skins of a rare or threatened animal. Beauty today goes hand in hand with ethical moral and responsibility.
There may be written or unwritten rules of what beauty should be, but personally I find beauty in things that are genuinely honest, true and real.
There is nothing beautiful in fake perfection, pretentiousness or overly staged styling. Sometimes less is more.