The contrast between light and dark that was first brought up by the "starry skies" in line 2 is repeated and developed in line 3.
Everything that is great about both "dark" and "bright" come together in this woman. Essentially, she's got the best of both.
Her "aspect" can mean both her facial expression and her overall appearance.
So her whole appearance and especially her "eyes" create some kind of harmony between "dark" and "bright."
If this seems weird to you, think of a really beautiful person who has dark eyes that always seem to sparkle – or someone whose eye color contrasts with his or her hair color in an attractive way. That's what Byron's talking about – contrast that creates beauty and harmony.
Byron's setting up a binary, or opposition, between "bright" and "dark," but it's important to realize that neither is considered better or worse than the other. Both have aspects that are "best.