What is the major theme of the novel Under Western Eyes?

Under Western Eyes is a novel by Joseph Conrad. The book was published in 1911. The story is set in early–twentieth-century Russia amid the brewing revolutionary movements of that time period. Six years after the novel was published, Vladimir Ilyich Lenin launched the Bolshevik Revolution and overthrew the government of Alexander Kerensky, ushering in the beginnings of the Union of Soviet Socialist Republics (USSR).
The main theme throughout the novel is that the common man—exemplified by the "regular" citizen Razumov, who does not participate in politics—is thrust into the greater drama of the national political landscape whether he likes it or not. Throughout the novel, Conrad subtly shows the duality of regular people living their normal lives and the intense political and social forces happening around them.
Razumov's meeting with the revolutionary terrorist Victor Haldin shows how our lives can change with a random chance encounter. When Haldin dies indirectly due to Razumov's actions, the political spirit of Haldin seemingly reincarnates within Razumov. Even when he admits to Haldin's family and revolutionary friends that he lied about his relations with Haldin, Razumov has transformed into the person he's been pretending to be.
At the end of the novel, when the revolutionaries who assaulted him make amends, Razumov becomes a political philosopher—a completely different person from the Razumov we saw in the beginning of the story.
The theme that Conrad wanted to reiterate through this ending is that we are shaped by the social, political, economic and historical forces around us, whether we want those changes or not.