One of the main themes in this story is "going native". This means that a colonist assimilates with the natives and starts being one of them.
Maugham shows us in "The Force of Circumstance" that he thinks a white man must not "go native". We see this in the way how he describes Doris and how he describes Guy. While Doris always stays true to her British origin and manages to live her normal lifestyle in a completely different environment. Doris personifies the western values and rules in the time of colonization, she stays true to them and finally leaves with her intact dignity. This rules say among other things that British must not "go native", British have to be self restrained and more. Guys behavior on the other hand is not comparable to the western standard. He loses his self restrainment and falls in love with a native woman, she gives birth to his children and guy even starts to live like a native. His last try to hold on to British society is when he brings Doris to his outpost. This fails because Guy already has a wife and he can not just push her away and reject his responsibility for his first family.

The other theme is living in a colony.
The British in the colonies are ripped away from British society, and left alone in the wilderness of another continent. They try to stick to their behavior but living in another country is hard, therefore they have to adapt to the surrounding. Like in the short story "An Outpost of Progress" from Joseph Conrad, also in this story the British settlers failed, they could not manage to live in the wilderness and to stay British in the meantime. The author seems to decline colonialism in general, he shows us that Brits would fail to live in a colony if they do not attune to the natives. This makes them no longer be a human, according to the view of European society in the colonial time.