A theme is an essential idea or meaning explored in a narrative. There could (and usually are) several in any given novel. One of the themes developed in Levathin's Every Day is the search for happiness. Interwoven into the theme of happiness are the themes of homosexuality and personal identity.
The overall theme of the book is love; however, the real theme is much deeper. The book is written from the perspective of A, who is neither male nor female, homo- or heterosexual. A is described by the author as "purely self." A is an entity which jumps from body to body each day adopting each new self as his (I'll use the masculine pronoun just to make it easier) own for the day, experiencing religions, ethnicity, and society through different eyes.
The ability of A to impose himself into different bodies is getting closer to the crux of the theme. Can a being (or person) put aside all notions of society and interact freely with everyone else OR are people so hardwired into social constructs that conflict is inevitable?
The theme would end there if not for A falling in love with Rhiannon. This brings a new complication to A's life. Can Rhiannon love him in return despite the shifting physical and social typecast of life? As a gay author, Levithan uses this as the basic theme of the book. Is love constrained to remain defined by the traditional notions or can it transcend race, gender, nationality and even personality? If love can transcend the notion of society, then can people who truly love accept everyone as a being, or self, rather than as white, black, Christian or Jewish?