What is the theme in To His Excellency General Washington by Phillis Wheatley?

Freedom is the main theme of the poem.

Freedom

The date that Wheatley wrote this poem, 1776, is familiar to Americans with even the weakest sense of history as a date associated with freedom. It is mentioned every year on one of the nation's major holidays, the Fourth of July, as the date of the signing of the Declaration of Independence, which proclaimed that the colonies were no longer subject to the rule of England. In the poem, Wheatley not only asserts that America has a right to be free from British rule but goes so far as to identify America as the land of freedom.