Clearly, any reading of this novel identifies racism as a central theme, but I think that Paton is actually exploring something deeper than the symptoms of the situation we are presented with. Paton seeks to explore the underlying cycle of inequality and injustice which itself leads to racism. This is his central focus and will help you to perhaps concentrate on the causes behind racism rather than racism itself.
Consider the setting of the city of Johannesburg - a place with enormous inequalities which, although not excusing the behaviour of characters such as Absalom and Gertrude, clearly help us to understand the desperation of such characters and how they are driven to desperate means. Inequality results in black S. Africans being allowed to have less land than whites, resulting in over-farming and lack of food. This in turn drives them to seek employment in Johannesburg. Yet characters such as Gertrude and Absalom are forced to realise that this emmigration dislocates them from the safety and protection of their tribe, and they are forced to turn to crime in a context of limited opportunities.
Of course, their stories are echoed throughout the novel which is set in a city with massive slum neighbourhoods which are characterised by violence against whites. Raging against their situation and trying to gain wealth quickly, blacks rob whites who in turn become paranoid and forces them to villainise the blacks, robbing them of any sympathy. This in turn makes the conditions for the blacks worse, making them angrier and more determined to lash out. Thus the cycle is perpetuated. Both parties justify their actions as natural and reasonable responses to the stance taken by the other group - Absalom's lawyer, for example, tries to claim that Absalom is a mere victim of society. Such an approach makes understanding between these different groups all but impossible and paints a bleak future.