There are a number of prominent themes in this novel. Forgiveness is one theme, coming into play in the second half of the novel. Another theme that applies largely to the second half of the novel is personal growth through personal acceptance.
Celie learns to become herself, so to speak, when she learns to accept her preferences and to see value in who she is (the way she is). Ironically, this acceptance leads to big changes in Celie's character. She grows stronger, more vocal (becoming outspoken), and she finds that she is able to let go of much of her bitterness and hate.
Where she had hated Mr. Albert _____, she comes to pity him and connect with him. In the end, the two are friends. This change in their relationship is a direct result of Celie's changes and Albert's changes.
Albert's changes are driven by loss, as he is left behind by everyone in his life. He comes to realize that he can open himself up, soften himself, and not be hurt. He learns to love, he says.
Celie does the same in her own way.
For both characters, self-acceptance leads to an ability to love. The ability to love, in turn, leads to a willingness to forgive. Celie forgives Albert and Shug, through love and (critically) through a confidence that she will be alright with or without them. Forgiveness becomes safe when she achieves confidence in her own value.