Throughout "She Walks in Beauty," the speaker draws on the symbolic connotations of light and dark to evoke the beauty of the woman described. In the first stanza, the speaker says that
all that's best of dark and bright
Meet in her aspect and her eyes.
Darkness can be a symbol of mystery, and light is often a symbol of purity. By saying that this woman has "all that's best of dark and bright," the speaker implies that this woman's beauty is at once pure and also mysterious.
At the end of the first stanza, the speaker also says that the beauty of the woman is like
a tender light
Which heaven to gaudy day denies.
The implication here is that the woman is so beautiful that she radiates with a heavenly, angelic light which makes the ordinary light of day, by comparison, seem garish or "gaudy." The "tender light" thus symbolizes the woman's angelic beauty. The fact that this woman's beauty is angelic is perhaps why her beauty is also, as noted above, somewhat mysterious.
In addition to the symbols of light and dark, the "starry skies" symbolize the aforementioned ethereal, heavenly beauty of the woman, and the "cloudless climes" symbolize the woman's unspoiled beauty. Her beauty is unspoiled, just as a clear sky is unspoiled by clouds. In the final line of the poem, the speaker describes the woman's "heart." The heart is often used as a symbol of love and goodness, and the speaker uses it here to evoke how lovely and good this woman is. At the end of the final line, the speaker says that this loveliness and goodness stems from the woman's "innocence."