The phrases build up the theme of contrast and balance in beauty in this poem of Lord Byron are:
"And all that’s best of dark and bright
Meet in her aspect and her eyes."
This lines seek to show that beauty is not always lights. In order for something to be beautiful, it has to have both positive and negative aspects. Nothing is fully good or fully bad, it has to posses a balance, an equilibrium in which all the parts come together to create a truly beautiful thing.