Two important themes in Beka Lamb are the issues of colonialism and racial conflict. During the time of the novel, Belize was struggling for its political identity. There are five distinct races identified in the novel and in Belize at the time, and they are all struggling to get along with each other.
In chapter 15, Bill Lamb has a discussion with his mother, Granny Ivy, over what shirt he prefers to wear to work. His mother has ironed a shirt for him that is in the style of the "Guats" - people from Guatemala. Guatemala was trying to take over Belize and re-institute Spanish colonialism. Bill does not want to wear this shirt for fear it might indicate political leanings that he does not have. The PIP (People's Independent Party) group to which Granny Ivy belongs favors an alliance with Guatemala to solidify Spanish influence in Central America. Bill Lamb is not happy about British Colonialism in Belize, but he has come to accept it. He fears that if the British leave, Belize will fall into anarchy. He tells his mother:
Hatred of British colonialism unites us now. There are so many races here I wonder what will keep us together once they leave.
Emilio represents the Spanish influence in Belize, and Toycie represents the Creole. This is the chapter where Toycie tells Beka she is pregnant. This situation also represents the oppression of the Spanish against the Creoles, as Emilio will later refuse to marry Toycie.
In chapter 16, Emilio tells Toycie he cannot marry her, fulfilling Beka's earlier prophecy that Bakras do not marry "Creoles such as we." This chapter intensifies the theme of racial and political conflict of the prior chapter.
You can read more about the themes right here on eNotes.