Mistaken Identity
Early in the film we see the Tramp meet the Flower Girl and receive a flower from her, but when a wealthy man walks by and gets into his car, the girl believes that the fancy car door she hears belongs to the Tramp. The Tramp does not say anything, and so takes on the identity of being a much wealthier man, in spite of being very poor. When the Flower Girl goes through a procedure to restore her eyesight, she expects that when she meets the Tramp one day, he will be a wealthy and distinguished man. However, she discovers by the end that he is in fact a poor tramp in tattered clothes. In this way, the Tramp's mistaken identity is central to his romantic plot line.
Additionally, the Millionaire has an inconsistent perception of the Tramp's identity. When he is drunk, he instantly recognizes the Tramp as the man who saved him from committing suicide, but when he is sober, he has absolutely no idea who the Tramp is. Because of this misunderstanding, the Tramp ends up getting wrongfully arrested and put in jail for robbing the Millionaire, even though he did not. Thus, we see that throughout the film, a thematic center of the Tramp's journey is mistaken identity, the fact that people often misunderstand who he is.
Class
The Tramp strikes up an unlikely friendship with the Millionaire when he prevents him from killing himself. The only problem is that the Millionaire only recognizes the Tramp when he is drunk. Only through intoxication can the wealthy man reach through the class difference between the two men and extend a helping hand. The difference in class is literalized by its only being perceptible through drunkenness, a comment on the disingenuousness and shortsightedness of the upper classes.
Additionally, the woman with whom the Tramp falls in love, the helpless Flower Girl, is a member of the lower classes, forced to sell flowers to make rent for herself and her elderly grandmother. We see her struggles with money, and her desperation to make rent before getting evicted. Posing as a wealthy man, the Tramp offers to help her. There is a sense that she trusts him more because she thinks he is wealthy, but she also assures her grandmother that it is not only his wealth that entices her. By the end, she is much more well-off, while the Tramp is in tatters, but when they touch one another's hands they are reminded of the romantic connection they built many months before. The film ends with an image of two people from very different class situations reaching out and feeling connected to one another.
Love and Beauty
The Flower Girl is a representation of beauty, innocence, and love throughout the film. She sells flowers, small tokens of beauty and life, and thus we see that the beauty she represents, and the love she stirs in the Tramp's soul, is an important theme in the film. During the boxing match, an image of the Flower Girl appears to the Tramp in his corner after taking a beating, and the thought of her gives him the strength to carry on. Additionally, the final scene is a tribute to beauty and love, and the connection shared between two people, however unlikely a pair they may be. It is only when the girl touches the Tramp's hand that she truly recognizes him and feels affection for him—not because of his wealth, but because of his beautiful heart.
Tragedy vs. Comedy
The film is highly comedic, with many slapstick routines and wild physical antics, but it also explores some dark territory as well. For instance, the Millionaire is suicidal when the Tramp first encounters him by the river. Deliriously drunk and despondent, he ties a large stone to his body with plans to drown himself. The way that the Tramp tries to save him quickly becomes an absurd slapstick routine all its own, with each of the men falling into the water over and over again. In this way, Chaplin makes light of the theme of suicide and violence, by showing its inherent absurdities, the ways that it can so easily become a routine, but he also suggests that underneath the lighter and more laughable moments of life are some darker impulses. The Millionaire vacillates between deliriously happy and assuredly suicidal, and these shifts represent a major theme in the film: life can swing back and forth between the tragic and the comic.
City life
After all, the film is called City Lights. While the narrative does not directly address the particularities of urban existence, the entire plot is informed by its taking place in a city. Characters run into each other, fall into unexpected scenarios, and get into wild adventures that have a lot to do with the diversity and activity of city life. The rich and the poor live in close proximity, and their lives become entangled, which reflects the particularly congested and manifold quality of city living. Living in the city is an important theme in the film.
Suffering & Challenge
A major theme in the film, in spite of its comedic luster, is suffering, particularly the suffering of the Tramp. While other characters, like the Millionaire and the Flower Girl, suffer in their respective ways, none have such a unique mode of suffering as the Tramp. Indeed, as a "tramp," Chaplin's character is defined by the fact that he suffers by living outside of society, never quite able to integrate himself into the wider world. As a result, he is out of step with nearly every event in his life. One minute he is getting water thrown in his face, the next he swallows a whistle, the next he is eating streamers that he thought were strands of spaghetti. Indeed, most of the humor of the film stems from the Tramp's ability to suffer so expertly and comedically. The viewer is meant to laugh because they cannot believe what the Tramp has to endure. Every activity is a challenge, but because of Chaplin's virtuosic comedic chops and physical abilities, these challenges become beautiful performances, irresistible "bits" that allow the viewer to laugh.
Loyalty
When the Tramp saves the Millionaire's life, the Millionaire becomes outrageously elated, and professes his loyalty to the Tramp. He declares that the two of them will be lifelong friends, and talks a big game about his affection for the hapless Tramp. Thus it is especially ironic when the Millionaire has no recollection of the Tramp when he is sober, and treats him as he would any other vagrant. This inconsistency shows that the film is seeking to poke fun at and question the nature of loyalty. Meanwhile, the Tramp, who makes no grand claims of loyalty, is steadfastly devoted to the Flower Girl. These two different characters represent very different approaches to the notion of loyalty.
