The main theme of this poem, not surprisingly, is love. In fact there's really not much other than love going on in this poem.
In the poem, written in 1845 while she was being courted by the man who would become her husband (the English poet Robert Browning) she expresses her love for him in various ways.
To the extent that the poem is about something other than love, it is able to tell us something about her beliefs. We can see her beliefs by looking at what she compares her love to. She talks about the ability of her soul to love (Christian belief) and her belief that she will love him even after death. She also talks about loving him like men strive for right, implying she believes people try to be good.
But overall, this is a love poem and nothing else.