A number of themes are explored in "City of Glass," including homosexuality, love, Christianity, authority figures and other religious beliefs. Prejudice is also a major theme in the book. "Shadowhunters," for example, are suspicious of certain groups of people, including vampires that can tolerate sunlight.
Love is one of the most important themes in "City of Glass." The story focuses on the journey of Clary to rescue her mother. It is illegal for Clary to enter the city where her mother is imprisoned, so this is a dangerous journey that requires significant sacrifice.
There are also a number of romantic relationships between characters throughout the book. One of the head vampires in the book, Raphael, uses the belief system of Christianity to convince the rest of his family that he isn't a vampire. By visiting a church regularly and wearing a cross, he manages to avoid detection. Other belief systems are also mentioned in the book, including the Star of David and Seal of Solomon.
Homosexuality is a theme that becomes important when the main character, Clary, sees a female character kissing another girl. Two male characters are also involved in a homosexual subplot in the book. Also, profanity is common in "City of Glass."