The title gives away the main theme of this essay: in it, Mary Wollstonecraft focuses on women's rights, arguing that women should be allowed to have the same educational opportunities as men and that denying them this opportunity inhibits their "usefulness" to society as well as making them unhappy. The strength of her essay is that she chooses to present women as human beings first, only considering their gender as a secondary issue.
Thus the main theme of this essay is that women should have the same access to education as to men so as to help all of humanity improve itself. Note her comments on the current state of affairs:
One cause of this barren blooming I attribute to a false system of education, gathered from the books written on this subject by men who, considering females rather as women than human creatures, have been more anxious to make them alluring mistresses than affectionate wives and rational mothers...
Note the alliteration and imagery in the "barren blooming" of women that the author sees. Denying women a "proper" education has meant that women are only "anxious to inspire love" when they should be focusing their minds and talents on nobler and more fitting aspirations.