Sure! there are quite a few other themes in the book! For example, you could take the theme of opulence vs poverty. You could also talk about the theme of greed and selfishness or the theme of sexism (in regards to the poor treatment of women).
How about the theme of opulence vs poverty in the characters of Nanga and Odili? Look at the affluence exhibited in Angus Palace! This can be vastly contrasted with the poverty of Odili who refuses a job with the government in order to be a plain teacher in a rural area.
In regards to greed and selfishness, it would be easy to portray those themes and the character of Odili because he succumbs to both greed and selfishness even through his teaching. Poverty does not bring out the best in people, it seems.
Or how about the deplorable treatment of women? Look at Nanga's supposed parlor wife named Edna. Edna's poor treatment, in addition to the treatment of the other servants in the household, shows that people in the upper echelons of society (especially those who don't know or understand their specific place) treat female servants with contempt.
Hopefully, this will get you started on a few more themes. Click the links below to find out even more.