What are some important themes explored in An American Visitor by Joyce Cary?

The primary theme in Joyce Cary’s novel is colonialism and its conflicting effects.
Another important theme is the relationship between personal emotions and moral convictions. Set in Nigeria in the 1930s, when it was a British colony, An American Visitor presents the story of Marie Hasluck, the titular visitor. An American journalist traveling in Nigeria, she unilaterally condemns colonialism as oppression and does not wish to be swayed from her utopian, idealistic vision of society. Believing wholeheartedly that the indigenous African peoples are capable of self-governance without any European guidance, she also refuses to admit that any of them have bad intentions.
Marie’s convictions are put to the test, however, when she falls in love with Bewsher, a British administrator. She is swayed by his insights into indigenous culture, which inform his administrative skills. Ultimately, however, her personal feelings cannot change things for the better, and the political situation takes a turn for the worse. While Cary does not imply that Marie caused Bewsher’s death, he shows that naïveté blocks understanding of the complexities of colonial rule.
https://www.nytimes.com/1961/08/27/archives/a-womans-way-in-darkest-africa-an-american-visitor-by-joyce-cary.html