As with any work of literature, there are a dazzling array of possible themes that you could identify when examining this excellent play. However one theme that to me stands out clearly is that of class differences and the conflict that this creates. Consider the main character, Jimmy. He seems to be a victim of the structure of the different classes that make life so difficult for the working class. In this world that we are presented with, getting ahead and success are dependent on things that appear to be largely out of your control, such as what kind of family you are born into. Thus it is that, although Jimmy worked hard to get a degree and therefore has a good education, the fact that he was born into the working class and went to the "wrong" university means he is limited in terms of his options. This is in sharp contrast with Nigel, who, in spite of his obvious deficiencies as a character, is a Member of the British Parliament and will obviously do well in this world.
In addition, consider the presentation of Alison's mother, who does everything she can to prevent her daughter marrying Jimmy. The only character who appears to be unmarred by the pervading class distinctions that delineate so many characters in this play is Cliff, who seems to be able to accept his own position and identity by declaring honestly that he is "common," in sharp contrast to others who struggle with their own sense of identity and rage against the way that class is used against them.
The class system is therefore presented as a divisive system that disempowers and empowers depending on the accident of birth, rather than any other basis, such as individual merit. It is this that Jimmy rages against, producing the stereotype of the "angry young man" that dominates this play.