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.