What is the theme of Emily Dickinson's poem "How Happy is the Little Stone"?

Despite its surface simplicity, Emily Dickinson's poem "How Happy is the Little Stone" is thematically complex. In the poem, the narrator is reflecting almost wistfully upon the happiness of the stone she is portraying. As the narrator is human and the stone is not, this brings up the first theme which is a contrast between humanity and nature. 

From a religious point of view, Dickinson's contrast between humanity and nature invokes the theme of Original Sin, in which humanity is fallen and nature is not. Thus the stone is described as "Fulfilling absolute (i.e., divine) decree" simply and easily in contrast with humans, who must struggle to live a moral life due to their fallen nature.

Another theme is the virtue of simplicity. The stone lacks human vanity, and it does not need to worry about choosing clothing or having a career. Unlike humans, who must struggle to make choices in their lives, the stone is personified as content with its own nature. Thus the nature of the stone suggests that we might be happier if we attempted to emulate the simplicity of the stone.

This poem by Emily Dickinson suggests that it is much easier and more enjoyable being a small, inconsequential object overlooked by everybody else than it is to be human and to have all the responsibility and obligation that comes with that identity. A little stone, Dickinson suggests, doesn't have to worry about "Careers" or the various "exigencies" that plague us as humans. Note how Dickinson ends the poem to cement her argument that the "little stone" is much happier than humans:

And independent as the Sun
Associates or glows alone,
Fulfilling absolute Decree
In casual simplicity—

Dickinson implicitly sets up a comparison between the life of a human and the life of a stone, and finds the life of a stone much easier and more simple than the life of a human. A stone is able to fulfill its "absolute Decree / In casual simplicity--", or fulfill its purpose simply and without any confusion. This poem suggests that the life of a human is far more complex, and that the speaker is struggling to do exactly what she envies that stone for doing so easily. The stone is independent of itself and able to fulfill its purpose without the various worries and concerns that plague humans. Dickinson sees this and finds herself intensely envious of this simplicity.

What poetic devices did Emily Dickinson use in "How Happy is the Little Stone?"

The main poetic device in "How Happy is the Little Stone" is personification. Personification is a figure of speech that gives human qualities to animals, objects, or abstract things/ideas. The stone is given human qualities to illustrate how the stone, or we who observe the stone, would consider stone's purpose in the universe. 

The stone is "happy" because it has a carefree existence. It rambles without care for a career or some urgent (exigent) needs. The speaker of the poem gives the stone more human qualities. It "rambles," doesn't "care," and never "fears." 

The speaker uses a simile (a figure of speech using "like" or "as" to compare two different things) to compare the stone's independence to that of the sun's independence. The speaker envies the stone's independence, again treating the stone as if it were human (or conscious). 

This poem is an ode. And ode is a lyric poem that praises or glorifies some aspect, quality, object, or being. This is usually something in nature. The speaker praises the stone for its simplicity and immutability. Relative to mortal things like a flower or a human being, the stone changes very little and lasts a lot longer. The speaker longs to be like the stone because the stone does not have to deal with human cares and concerns. The benefit of being like the stone is that existence would be carefree. The downside is that the life of a stone is a thoughtless existence; so, you might not be conscious enough to know you were carefree. A great poem that illustrates the aphorism "ignorance is bliss."