I would say that love is a major theme. That really shouldn't be a surprise, because love is a focus of just about everything it seems. Crystal Kile summarized it this way: "The belief [in true love] is everywhere and the fact that about 90 percent of Americans marry at least once suggests that our faith in the belief that one true love is the key to human happiness is as strong as ever."
The novel, "Weep Not, Child" portrays a pure love between Njoroge and Mwikahi. Their love is portrayed as that love that is an all powerful force that can overcome anything, which is good since the novel is full of pain, suffering, social class differences, and in-fighting. Love over coming social class differences is key to their relationship since Mwikahi is from a very wealthy family and Njoroge's famiily can barely afford school.
I think a second major theme is the importance of family. It ties in with the love theme as well. Njoroge and Mwikahi want to run away together, but decide not to. They feel a larger sense of obligation to care for their family members. Another example of family loyalty is Boro who feels that it is his duty to his family to seek revenge for his father's death. He believes that will honor his family and provide a better future for them as well.
Using Njoroge and Mwikahi's relationship sheds light on a third theme of the story. Thematically, social class is a big part of "Weep Not, Child." It's one of the biggest obstacles to their relationship. The novel does a nice job of showing the struggles of both the poor and rich social classes.