What are the theme, style, and tone of Robert Frost's "Design"?

Perhaps echoing the words of Hamlet, "Nothing is either good or bad, but thinking makes it so, Frost's superbly constructed sonnet, "Design," underscores the observation that perception often determines reality for an individual as a theme.

While at first the Italian sonnet of Frost, in its light tone of near cajolery in its observation of nature, suggests the poetry of the Romantics, the poem moves to a Dark Romantic's metaphysical wrestling worthy of Melville's Ahab with its debate upon the goodness or evil of white. Then, too, there is the overtone of the New England Puritan in the consideration of a universe "designed" with the moth, the spider, and the flower all being white.  Further, the sestet ends with a question worthy of the Naturalist Stephen Crane:

What but design of darkness to appall?--
If design govern in a thing so small.

Thus, Frost's speaker proposes the question of whether the universe is benign, malign, or simply indifferent with a tone that moves from humorous observation to one of pondering the meaning of existence: 

What had that flower to do with being white?
The wayside blue and innocent heal-all?
What brought the kindred spirit to that height?
Then steered the white moth thither in the night?

Critics feels that this poem is more perfectly composed that most of Frost's as the figures in the mind match those of the ear.  For instance, the "daring" use of the same end-rhymes echoes the persistence of the speaker's mental debate. And, the disturbing discovery in the mind comes with the rhyme in line 9 of "height" and "night" with the realization in the speaker's internal monologue that what should represent "good" may, in fact, be "evil" or simply indifferent and perception may, indeed, be only the individual's reality.

What is the line-by-line explanation and theme of Robert Frost's "Design"?

Robert Frost worked on the poem “Design” for several years before first publishing it in 1922, and he tweaked it afterwards, ultimately including it in his 1936 collection A Further Range. The poem is a deceptively simple 14-line sonnet that takes up a huge question as its theme: if there is intelligent design in the universe, then why does that design include dark and dangerous forces? Frost uses the example of a spider perched on a flower, having caught a moth. Yet the implication is much deeper than that, obliquely raising questions about the existence of evil, pain, and destruction.

The poem is divided into two parts, like many sonnets are. The first eight lines describe a “dimpled spider, fat and white,” perched on a type of flower known as the “heal-all, holding up a moth” with white wings. The whiteness of the spider echoes the whiteness of the moth, yet at the same time contrasts with the metaphorical darkness of its act (killing the moth). Ironically, the moth has been killed and will be eaten on a flower used in traditional medicine to heal a variety of ailments.

This irony is not lost on Frost, who notes that the three elements are “mixed ready to begin the morning right,” while also comparing them to the “ingredients of a witches’ broth.” On one hand, the spider has killed and will consume the moth simply because that is the nature of this lowly creature. This nature can’t be judged as unethical. On the other hand, Frost insists that readers see the act as essentially violent and dark: it is, in any case, a killing.

This perspective prepares readers for the final six lines of the sonnet. They explicitly ask what the healing flower had to do with the business of the spider and moth, and raise the question

What brought the kindred spider to that height,
Then steered the white moth thither in the night?

Why, in other words, did the moth have to die? What force compelled this? The laws of nature? A god? Frost registers shock that a “design of darkness to appall” would “govern a thing so small” as a spider killing a moth, consequently questioning the very essence of that design.