Question

I am trying to figure out the theme description of the following excerpt from Christina Rossetti's "The World". By day she wooes me, soft, exceeding fair;
But all night as the moon so changeth she;
Loathsome and foul with hideous leprosy
And subtle serpents gliding in her hair.
By day she wooes me to the outer air,
Ripe fruits, sweet flowers, and full satiety:
But thro' the night, a beast she grins at me,
A very monster void of love and prayer,
By day she stands a lie: